Accept directory as argument to cdn

cdn will `cd` to a valid directory as first argument
Fix adding individual files to tag list

Each file in the tag list must appear in a separate line. This patch
appends files by iterating over the expanded file list, and appending
each of them in a separate line
Introduce functions and bindings for using lgrep

Default mappings for C-n and C-p are now changed to do traverse local
quickfix lists.