Add defcustom to configure a path between host and filename

[jvtrigueros]

I ran into an issue where my remote type was using gitiles type, but between the host and the filename there was a custom path. At first, I tried to solve this with add-advice but it quickly became a mess, so it was much easier to provide this as an option.

browse-at-remote-append-path-to-host allows users of the package to customize the remote URL if needed:

<web-proto>://<host><path><filename>

This is my first PR to an emacs package so I'm not sure if this is the best way to go about it.

[cpaulik]

Could this be generalized to a full host replacement?

I'm looking into a fix for #70