Fixes #15 - Added profile parameter for conditional profiling.

Added the --profile parameter for specifying conditional profiling
parameters, for example:

--profile="os: rhel; condition: user"

When used in conjunction with Publican 3.2+ all DocBook 4.x profiling
conditionals are available. The DocBoook 5 "audience" profiling
conditional is converted to "condition" as part of the XSL transform
step. Users wishing to use the audience conditional should instead use
"condition":

--profile="condition: user"