Regular expression support

[hairyhenderson]

It'd be useful to have a sort of sed capability inside templates... Probably some simple wrapping of regexp.ReplaceAllString.

The usage should be something like regexp.Replace "expression" "replacement" "input":

$ gomplate -i '{{ "1.2.3-59" | regexp.Replace `-([0-9]*)` `.$1` }}'
1.2.3.59

I'm not sure about the function name though. replaceAll is taken and doesn't handle regexes - maybe we could just modify replaceAll to support regexp instead, though that would certainly break some users. I'm not sure I like regex.Replace, but... 🤔