yamllint

This adds yamllint checks for helping users understand issues with their metadata files. (as suggested in #745 (closed))

• yamllint is added as an optional dependency. So if it's not installed checks will be skipped (silently)
• checks are only taking place in 4 cases:
  • when pyyaml fails to parse any .yml srclibs or app-metadata file
  • when fdroid lint is gets called app-metadata files and all their references srclib files
    (running yamllint for all .yml srclib and app-metadata files increases execution-time by an order of magnitude. On my test environment from 8.77s to 104.99s. So when running fdroid lint without a specific list of appids yamllint is off by default. I've added an option to run it regardless: --force-yamllint)

Here's an example of how yamllint output will look like:

$ fdroid lint --force-yamllint
metadata/io.anuke.mindustry.yml:187: trailing spaces (trailing-spaces)
metadata/io.anuke.mindustry.yml:189: trailing spaces (trailing-spaces)
metadata/net.olejon.mdapp.yml:155: trailing spaces (trailing-spaces)