Add custom GitHub URL support

[tg-m]

Hi,

I've noticed that the GitHub URL is hard coded. It prevents pushing to repositories if there are multiple GitHub accounts configured on the same machine, e.g., if ~/.ssh/config look like this:

Host github.com
    Hostname github.com
    User git
    IdentityFile ~/.ssh/main-user/github/id_rsa
    IdentitiesOnly yes

Host github.com-other-user-url
    Hostname github.com
    User git
    IdentityFile ~/.ssh/other-user/github/id_rsa
    IdentitiesOnly yes

The commit/push with custom URL can be invoked as:

ablog deploy -g other-user --github-url="git@github.com-other-user-url" -m "..."

[nabobalis]

Thanks @tg-m