Ability to skip vendor and specific git remote name

[skymeyer]

Introducing -include-vendor flag to generate vanity URLs for vendored packages, off by default. Adding ability to override the default "origin" remote name used to determine the URLs.

Fixes #4 and #5

Usage:

goredirects -include-vendor bramp.net outputdir
goredirects -git-remote=upstream bramp.net outputdir
goredirects -git-remote=upstream -include-vendor bramp.net outputdir

We maybe should abstract out the package and use a separate cmd using https://github.com/spf13/cobra for more user friendly CLI flag parsing. This will allow to embed this package in other logic as well.

[coveralls]

Coverage decreased (-0.7%) to 8.943% when pulling a9c39f78de130631aa6a1b97e21d428691e6e7b6 on skymeyer:flags into b3920ba1acb5c1709b71e76034c05531b32205e6 on bramp:master.