Allow configuring Git remote used to generate meta resources

[nohuhu]

When Git remote pointing to the master repo is not origin (say, upstream), Minilla will try to override meta resources with Git URLs generated from origin remote.

Allowing to configure what remote is used solves this problem.

[Grinnz]

I'm not sure this is the optimal way to configure this. Remember the minil.toml is committed, and so needs to work for the owner as well as any contributors, who may have different remote setups.

[nohuhu]

Right, I was writing a comment describing the changes and realized that this might be the case. Any input on how to make this configurable on local basis? Environment variable perhaps?

[Grinnz]

There's two ways it could be configured in minil.toml that would work for all users. Either setting the github user (or perhaps user/repo) to override the one found in the origin remote (only if it's a github URL, of course) or setting the URL to use as a whole.

[nohuhu]

I see another way: add optional and non-committed minil_local.toml that would override config values in the main minil.toml. That would make it easy to change any values without conflicting with other people's repo forks.

@tokuhirom Thoughts?