Let ungron speak tabs too, not just spaces

[nigeltao]

I have my own command line tool that produces output that is (1) ungronnable, but also (2) human-readable. To aid (2), I'd like the equals signs to line up, if the Left Hand Sides aren't too different in their length:

foo.bar    = "val1";
foo.hijklm = "val2";
foo.quxz   = "val3";

The easiest way to implement (2) in my tool is to print a tab character '\t' instead of a space ' ' before the equals sign. But gron -u doesn't allow that, unless this feature request is implemented. Which may be as easy as, in ungron.go, replacing these:

l.acceptRun(" ")

with this:

l.acceptRun(" \t")