Adding replace function

[jakebellacera]

This patch adds support for a replace function that replaces text using regex which was previously discussed in #1511. I believe that this function should be in the core library because string replacement is a basic feature that has a multitude of use cases.

replace(subject, pattern, replacement, flags)

This function returns a String.

• subject - (String) the string to search and replace.
• pattern - (String) the pattern to search for.
• replacement - (String) the string to replace the matched text.
• flags - (String, optional) string of regular expression flags.

Use case: high-DPI image mixin

.hd-background-image (@src, @size) {
  background-image: url(@src);

  @media only screen and (-webkit-min-device-pixel-ratio 2, min-resolution 2dppx) {
    @hdSrc: replace(@src, "\.([a-zA-Z]{3,4})$", "_2x.$1");

    background-image: url(@hdSrc);
    background-size: @size;
  }
}

The mixin above will load the normal background image for normal DPI devices and the background image with _2x appended to the filename for high DPI devices.

[lukeapage]

I'm ok with this. If no-one complains I will pull into the next release.

[jakebellacera]

Great!

[jonschlinkert]

Love it! I think it's great