Allow newlines in tags and variables.

@shopify/liquid for review
cc @bgotink & @Phrogz

## Problem

It was brought up in issue https://github.com/Shopify/liquid/issues/216#issuecomment-33605401 that liquid wasn't allowing newlines in tags and variables.  This could be helpful as a way to avoid unnecessary spaces and newlines.

I don't see any reason why we wouldn't support this.

I found out that the reason this wasn't working properly was that we use `/./` to match any character, but ruby documents this metacharacter as follows (http://ruby-doc.org/core-2.1.1/Regexp.html)
> /./ - Any character except a newline.
> /./m - Any character (the m modifier enables multiline mode)

## Solution

Add the multiline modifier to any Regexps that contain the `/./` metacharacter, since they all seem to be used to match any character.