CTAGS support

[cliffclick]

Currently hand-rolling some variant of
ctags -e --recurse=yes --extra=+q --fields=+fksaiS $(SOURCE_FILES)
Would like a gradle built-in, that auto-runs after e.g. a successful gradlew build
Will probably tweak the visited file list.
Will surely add a x.ctags support in the future to teach ctags how to parse .x files.