Allow pattern to be a function

In some cases, the pattern matching logic for which files to instrument
is more complicated than what can be easily added to a regular
expression. In these cases, you can set a function as the pattern. This
function will get executed with the filename and can perform any user
logic to determine if the file should be instrumented.
Updating docs to have correct filter option format
Updating pattern comment
Merge pull request #195 from tmrudick/master

Allow function as pattern value