features: Add FEATURE() and FEATUREMAP() functions.

Add two new dialplan functions: FEATURE() and FEATUREMAP().  FEATURE()
lets you set some of the configuration options from the [general] section
of features.conf on a per-channel basis.  FEATUREMAP() lets you customize
the key sequence used to activate built-in features, such as blindxfer,
and automon.  See the built-in documentation for details.

Review: https://reviewboard.asterisk.org/r/1871/

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@364437 65c4cc65-6c06-0410-ace0-fbb531ad65f3