support for Gitlab Releases as source

**Describe the problem/challenge you have**
There are projects which live on gitlab.com or internal gitlab instances. Those projects might provide downloadable assets on their release page. Vendir already supports assets from github.com based releases, but lacks support for Releases from Gitlab.

**Describe the solution you'd like**
Add a `gitlabReleases` source which works mainly like the `githubReleases` source.

**Anything else you would like to add:**
[Additional information that will assist in solving the issue.]

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