Show time-per-linter in verbose mode.

Auditors: chris

Test Plan:
Ran ../devtools/khan-linter/runlint.py -v in the webapp-root dir.  It
finished as expected, and gave time info like so:

   --- Running Pyflakes:
   user_models.py:3358: E=pyflakes= redefinition of function 'read_access_level' from line 3344
   1 errors (3.60 seconds)