Allow clients to specify placeholders when mapping keys for injection

Hello @nicks, @tessr, 

Please review the following commits I made in branch 'jamie-output-mapping-placeholders'.

f1c503d2242492b069e9b517651a414796309da3 (2014-01-27 14:44:21 -0800)
Allow clients to specify placeholders when mapping keys for injection

R=@nicks
R=@tessr