Add custom NFA logging to nfa_regexp.log

The goal is to get a ballpark estimate of the number of steps the regexp
engine undertakes in a session. One line of logging is done:

* Start of nfa_regmatch()
* For each character considered
* For each state of a character considered

Vimm should be compiled with -DDEBUG; if the NFA engine state graphs are
desired, then also use -DINCLUDE_NFA_DUMP.

This branch can also be found at:

https://github.com/guns/vim/tree/custom-nfa-log