Support for shortening file paths in qf/loc window

Long filenames can mess up formatting of qf/loc window, including truncation of the error message.

Suggestion is to add a filename shortening capability, in the spirit of [vim-qf](https://github.com/romainl/vim-qf).

For example:

`very/long/path/with/lots/of/subdirectories/filename.ext` -> `v/l/p/w/l/o/s/filename.ext`

...or with an option to specify number of characters to truncate (in this example 3):

`very/long/path/with/lots/of/subdirectories/filename.ext` -> `ver/lon/pat/wit/lot/of/sub/filename.ext`