config: add "replace" function [GH-1029]

[mitchellh]

Fixes #1029

This adds a replace function. From the docs:

  * `replace(string, search, replace)` - Does a search and replace on the
      given string. All instances of `search` are replaced with the value
      of `replace`. If `search` is wrapped in forward slashes, it is treated
      as a regular expression. If using a regular expression, `replace`
      can reference subcaptures in the regular expression by using `$n` where
      `n` is the index or name of the subcapture. If using a regular expression,
      the syntax conforms to the [re2 regular expression syntax](https://code.google.com/p/re2/wiki/Syntax).

[mitchellh]

This one is for you @phinze.

[phinze]

[phinze]

LGTM! I can barely keep up with you this morning! 😛

[mitchellh]

@phinze I've just been doing a bunch of easy things while you toil away at the hard things. ;) One of those "I need easier things" mornings. Haha.

[ghost]

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