support alternate separators for regex

[yshavit]

See: #277 (comment)

[yshavit]

Maybe something like::

!/#foo#?

• ! acts as modifier of what follows
• / says "the thing being modified is a regex"
• # is the separator char

One thing I like about this is that it generalizes to search-replace (#277) and quoted string separators (#56).

• !s#search string#replace string#
• !"#quoted string#

One thing I don't like about it is that it restricts you to a single-char separator. You couldn't really do something like !/##foo## because it would be ambiguous whether you wanted the separator to be ## and the contents to be foo, or the separator to be # and the contents to be #foo#.

But, maybe a single-char separator is good enough!

[yshavit]

Bumping this out of milestone 0.8: breaking changes & others because it wouldn't actually be an API change at all, just an app change. I originally had it as part of 0.8 to be a rider along with #277, but I'd like to get 0.8 cut, so I'm bumping this out.