Support downloading of non github packages

[richfitz]

Use full url, perhaps:

ropensci/lawn # comes from github
https://bitbucket.com/foo/bar # comes from bitbucket

Another option would be to use toml or yaml to break the file into different sources:

[github]
ropensci/lawn

[bitbucket]
foo/bar

[urls]
https://whatever....

But then, I don't have a need for this at present - for better or worse things seem pretty centralised on github...