[PATCH] Optional Required Attribute for Filter task

[Erik Meade <an...@eghm.net>]

We use the Filter task to do a lot of simple templating.  In some places
we use blank values, to "turn stuff off".  But in many places blank
values can cause problems.

Currently we setup a fail task to check required values are set.  This
patch adds an optional required attribute to the Filter task.  If the
required attribute is true, then a BUILD FAILED will result if the
value of the token is blank (value.equals("")) at the time of token
replacement.

Erik Meade