👍 Add GinBrowse command

[lambdalisue]

Users can visit the hosting service webpage (e.g. GitHub) of the repository.
This feature is powered by git-browse, so users can use equivalent options of that CLI command like

:GinBrowse
#=> Opens a blob page of the current buffer or a tree page of the current working directory in the HEAD commit of the current branch

:GinBrowse my-awesome-branch
#=> Opens a blob page of the current buffer in the HEAD commit of my-awesome-branch branch

:'<,'>GinBrowse
#=> Opens a blob page of the current buffer with selected lines in the HEAD commit of the current branch

:GinBrowse --path=README.md
#=> Opens a blob page of README.md in the HEAD commit of the current branch

:GinBrowse --path=README.md:10
#=> Opens a blob page of README.md with line 10 in the HEAD commit of the current branch

:GinBrowse --path=README.md:10:20
#=> Opens a blob page of README.md with lines 10 to 20 in the HEAD commit of the current branch

:GinBrowse --home
#=> Opens the home page of the repository

:GinBrowse --commit
#=> Opens a commit page of the current branch

:GinBrowse --commit my-awesome-branch
#=> Opens a commit page of my-awesome-branch branch

:GinBrowse --commit fd28fa8
#=> Opens a commit page of fd28fa8 commit

:GinBrowse --pr
#=> Opens a pull request page that contains the HEAD commit of the current branch

:GinBrowse --pr my-awesome-branch
#=> Opens a pull request page that contains the HEAD commit of my-awesome-branch branch

:GinBrowse --pr fd28fa8
#=> Opens a pull request page that contains the fd28fa8 commit

[lambdalisue]

Oops, I forgot to write documentations about g:gin_browse_aliases. I'll add it then merge.