Add Discussions API implementation

* Update list of available access levels

Update list of availabe access levels:
- Change `OwnerPermission` to plural
- Rename `Master` to `Maintainer`, see https://about.gitlab.com/2018/06/22/gitlab-11-0-released/#master-role-renamed-to-maintainer
  for more information

* Update access levels for backwards compatibility

Add option to list inherited project and group members. Updated the
documentation of the existing `ListProjectMembers` and
`ListGroupMembers` functions for clarity.

Fixes #484

* Query encoding should use url.PathEscape

In repository_files.go, url encoding is done with url.QueryEscape, but this makes it impossible to get files with a space in their name. Using url.PathEncodes changes the space encoding from "+" to "%20" and fixes the issue.

* Removed PathEscape when encoding project path

* Fix typos

And extend the MergeEvent struct.

* Add GetProjectLanguages to get languages from a project

* rename example fuction name

* Fix format issues, change type to fit better to the API

* Implemented access requests API

* Update API coverage in Readme.md

* resolve conversation

* added custom attributes

added tests

fixed typo

* Refactored to fit idioms of the package better

refactored methods to be  to have fully named functions

rewrote tests

updated typos and syntax

* fixed typo caught by linter

* refactored per maintainers request

removed blank line

Tick the `Search` box

* Implemented project badges api

* resolve conversation

Additionally updating existing *Options to include:
 - UpdatedAfter/Before - "Return merge requests updated on/before/after the given time"
 - SourceBranch- "Return merge requests with the given source branch"
 - TargetBranch - "Return merge requests with the given target branch"
 - Search - "Search merge requests against their title and description"

Corrected some outdated documentation.

More info here: https://docs.gitlab.com/ce/api/merge_requests.html#list-group-merge-requests

* projects.go: adding approvals_before_merge to CreateProjectOptions

* project.go: updating gitlab api doc ref to reflect change to CreateProjectOptions

* Implement DownloadSingleArtifactsFile

* Fix bad variable name

* Fix function docstring

* Make jobID an int var

* Use QueryEscape for consistency

* Dont escape filepath

* Fix typo

* Add weight field to UpdateIssueOptions struct

* Fix weight field order

* Update runners: add maximum_timeout option

* Update runners: add new methods and tests

* Update runners: fix typo on token

* Supplement Slack service

Refs:
- https://docs.gitlab.com/ce/api/services.html#slack-notifications
- #142

* gitlab: move BoolValue type and associated methods to gitlab.go

* services: update slack service: add url tags, fix typo, remove pointers in properties

* Revert renaming webhook field

* Fix ListProjectContributionEvents fails with 404 error (#449)

* Rename EventsService.ListProjectContributionEvents to EventsService.ListProjectVisibleEvents to match with gitlab API changes

* Adding missing straight option in repositories compare api

* Fixing struct reference for missing straight option

Adding support for 'list-merge-requests-associated-with-a-commit' (reference: https://docs.gitlab.com/ce/api/commits.html\#list-merge-requests-associated-with-a-commit) (#447)

* Adding Group Variables support

* Fixing naming per request

* Fixing merge conflict

* Fixing merge conflict

* Adding support for group issue boards

* Cleaning up comments and renaming GetGroupIssueBoardLists() to ListGroupIssueBoardLists() to match the gitlab API spec.

* Fixing comment lengths

* Adding Project Variable Support

* Adding missing semi-colons

* listtagopts added params

* test fix + gitignore