Introduce the `--devel` flag for `helm repo search` command.

`helm repo search` - searches only for stable releases, prerelease versions will be skip
`helm repo search --devel` - searches for releases and prereleases (alpha, beta, and release candidate releases)
`helm repo search --version 1.0.0 - searches for release in version 1.0.0
Signed-off-by: Mateusz Szostok <szostok.mateusz@gmail.com>