Added replace module, for multiple regexp match + replace against entire file's contents

This implements a multiline regexp replacement against the *entire* contents of a file. Unlike the `lineinfile` module, this would allow 1) matching more than one line of said  file and 2) replacing multiple matches. Currently, someone would have to implement this with a `shell` task using sed or perl.

The specific use case I built this for was to replace the entire `mpm_prefork` section from ubuntu's default apache config, while leaving the rest of the file in tact; this removes the section in question and replaces it with an include (to a file that I then created from a template):

```yml
- replace: >
  dest=/etc/apache2/apache2.conf
  regexp='^(?:#[^\n]+\n)*<IfModule mpm_prefork_module>[^<]+?<\/IfModule>'
  replace='Include prefork.conf'
  backup=yes
  validate='/usr/sbin/apache2ctl -f %s -t'
```

I was also considering adding an `all` boolean or `count` numeric parameter to control how many replacements to perform against the file -- it's currently just replacing *all* matches.