Add additional `list` parameters

[dylandreimerink]

Currently, list shows us the source code at the current location. This issues proposes to extend its functionality by adding some optional parameters. It would, for example be handy to pick how many lines above and below the current line you want to see, so specifying a count like list 40 would show 20 lines above and 20 lines below the current point instead of the default.

The second idea is to allow users to specify a start and end line number within the current or another file like: list 100-120 for the current file and list somefile.h:100-120 to specify some specific file.

Thirdly, it would be handy to be able to list a function, for example by saying list @func123, or a function within a specific file like list somefile.h@func123. Perhaps this can extends to symbols in general.