Make file writing multithreaded

Used to write a single file at a time, now it writes files based on the
number of CPUs you have (if you only have one thread it will be the
same, though).

When using this on `man --regex systemd` (95 man pages) it speeds up the
display significantly.
Need to catch all exceptions
Add manoption things

Lets you do things like specify the use of regex, etc.
Better documentation for writemans