use github.user instead of git.username

[ungoldman]

Reasoning: github.user is a preexisting standard (see https://github.com/blog/180-local-github-config), user.username creates unnecessary duplication of data in gitconfigs.

resolves moose-team/friends#160

Sidenote: +1000 points for "my voice is my passport" Sneakers reference.

[beaugunderson]

thanks for reporting and for the patch; I opted to make it backwards-compatible though (it now supports both places in 2.5.0) :)

[ungoldman]

Sounds good! Probably better not to introduce a breaking change :)