make auto escape optional

[bstaton1]

Currently, the usage of regular expressions like this are not supported:

match_p(post, "z[0-9]")

Because the [ and ] are auto-escaped using the ins_regex_bracket() function. It may be desirable to be able to use these characters in regular expressions like those presented here, as it would increase the generality of extracting particular nodes.

This may be accomplished by adding an escape = TRUE argument to all functions that call match_p(), which would also have the escape = TRUE argument. This would trigger whether ins_regex_bracket() should be called or not. The default should be TRUE to maintain compatibility with previously-written code.