[Feature Request] Enhancements for `veripress theme install`

[momocow]

Default branch

Currently to install third-party theme, command veripress theme install someone/the-theme is used. If no branch option is specified, veripress defaults to clone from master branch.

However, for some scenarios (like https://github.com/momocow/veripress-theme-suka), the third-party theme can be ported from other engine thus the repository is forked from others. To respect the original repository, the master branch is kept and a new branch is created, e.g. named veripress, which is the only place modifications occur. The default branch on GitHub is set to this new branch.

Given that all third-party theme repositories are hosted on GitHub, I suggest the branch option defaults to the default branch of GitHub, which can be retrieved from GitHub API within one HTTP GET request without any permission required.

It has not conflicts with most of the usage since the default branch of GitHub is default to master.

See https://developer.github.com/v3/repos/#get.

Third-Party Theme Prefix

Again as the example repository (https://github.com/momocow/veripress-theme-suka), it is named after veripress-theme-suka.

When installing, use the full name (i.e. veripress theme install momocow/veripress-theme-suka) to explicitly pin to a repository makes sense for me.

But we can use its short name (i.e. suka) without the prefix veripress-theme- after it is cloned. The sub-folder names (theme folders) of the local themes/ folder can also use the short names. It can be a little bit easier when specifying a theme to use, uninstall, or update while remaining its namespace on the online Git repositories.