fi(helm): add more tests for plugins

feat(tiller): update sprig to 2.7.0

This adds shasum, int, and float64 functions, and fixes quote escaping
for 'quote'.

Closes #1524

fix(helm): fix prompt error on Windows build

This backs out an earlier change that removed an int type assertion.

fix(hel): fix plugin format on tests

This adds tests for setupEnv, and also updates some documentation to
mirror the current state of plugins.

fix(helm): handle errors when plugin command is not found

fix(*): misc linter issues

fix(helm): add missing line ending on list output

Feature/repo url trailing slash

fix(ci): do not push canary image on release

chore(*): bump go1.7.3

fix(helm): write yaml without exec permission

feat(helm): Add Files.Glob method to permit file organization

Test plugins were in the original format. Updated to current format and
fixed the tests to stop silently ignoring the broken test.

If a 'command:' is not found for a plugin, it will not result in an
ExitError, but in a PathError. This prevents that condition from
panicing.

Closes #1609

fix(helm): add no-hooks to upgrade

Mark disable-hooks as deprecated.

Closes #1575

fix(helm): pass the no-hooks flag during upgrade

fix(helm): improve URL comparison logic

fix(Makefile): build windows amd64 binaries

fix(helm): fix doc string for ErrNotList

fix(tiller): validate names before performing operations

Normalize URLs before comparing them. This deviates slightly from the
URL spec, but in order to accomodate the predominant use pattern for
Helm. Specifically, './', '../', and '/' are all "interpreted" to be
filepath-like.

Closes #1588

This adds back support for Windows now that it has been tested by the
community.

Closes #1596

This just moves a var declaration to the top of a file and then properly
documents it.

Remove double explanation of why MariaDB is in the list