Support colon syntax in where modifier

[jasonvarga]

Since #5477 "fixes" the inconsistency with how modifier parameters work (#3614), I'm sure plenty of people using the where modifier as documented will run into an issue.

This PR lets the where modifier continue to work if you use the colon syntax:

{{ myarray where="foo:bar" }}
  ...
{{ /myarray }}

Technically that's a single argument of "foo:bar" which we'll just split.
If you do it the "right" way (where="foo|bar") it'll be considered two arguments ("foo" and "bar") and work fine too.

Tip: you can use the new modifier syntax to avoid all the confusion entirely:

{{ myarray | where('foo', 'bar') }}
  ...
{{ /myarray }}