[plugin installer] removed github support

[BigFunger]

Fixes #5591

• Removed default github url support. (functionality still supported by using the --url argument)
• Renamed all files using snake-case

[epixa]

Not a huge deal, but in the future would you mind doing a changeset like this in two commits: one for the removal of github support and the other for renames. It's easier to review each of those changes individually, and commits are a logical way to handle that.

[rashidkpc]

This LGTM from a code standpoint, but can you update the docs: https://github.com/elastic/kibana/blob/master/docs/plugins.asciidoc

[epixa]

This needs master to be merged into it

[rashidkpc]

Needs master merge, and still need docs

[palecur]

PR #5745 with the doc update is up.

[BigFunger]

jenkins, test it