[Feature Request] Open a specific file

[eval-exec]

Thanks for your git-open plugin, it helped me a lot. And I think it would be helpful to add open a file feature, for example:

$ ls
. .. .git src README.md

$ git open --file README.md # visit "https://github.com/[user]/[repo]/README.md" file on github

[skypesky]

done in skypesky/to-where-cli#45

npm cli: https://www.npmjs.com/package/to-where-cli