gitea support

[abs3ntdev]

this adds gitea support and a very rough framework for adding other host types. it adds two config values for host (git.example.com, gitea.whatever.foo) and host_type(github, gitea).

I tested it very briefly just by egetting some binaries from my personal gitea deployment so could use some additional testing.

example toml config:

host = "git.example.com"
host_type = "gitea"

example command line arguments:

eget --host "git.example.com" --host-type gitea user/repo

closes #61

[tarasglek]

This would solve #110 for me. It's too bad that it prepends api. to host= when hosttype=github

But even in current form, that's something that works for me