Support for module.exports functions

[filmaj]

This fixes #10, fyi @romainl @rkulla

[romainl]

Thank you @filmaj.

Could you add a test snippet to index.js and update the README.md to make this PR complete?

[filmaj]

Sure thing, I think I added what was necessary. I'm not 100% sure if the tags file output is correct - I see two entries related to the test function name mod_exports_func that I added:

➜ make && grep mod_exp tags
[ 2019-04-22 - 08:44:39 ] 'tags' file updated.
exports	index.js	/^module.exports = function mod_exports_func() {$/;"	M
mod_exports_func	index.js	/^module.exports = function mod_exports_func() {$/;"	F

Hmm, I guess it is also picked up as a method name - do you think that is OK?