Support ctags/etags generation

[jacereda]

This is my attempt at #890.

To try it with vi:

psc-docs --format ctags `find bower_components src test -name \*.purs -not -name externs.purs` `find bower_components src test -name externs.purs`  > tags

vi src/Main.purs
position the cursor on top of a symbol and press CTRL+], go back with CTRL+T.

With emacs:

psc-docs --format etags `find bower_components src test -name \*.purs -not -name externs.purs` `find bower_components src test -name externs.purs`  > TAGS

emacs src/Main.purs
position the cursor on top of a symbol and press ALT+.

[paf31]

I restarted the CI build, looks like Travis froze.

[coveralls]

Coverage increased (+0.02%) to 84.54% when pulling 95fd264 on jacereda:psc-docs-tags into db76716 on purescript:master.

[paf31]

👍 Thanks!