Added namespace mappings and RC file

Added a namespace mapping statement, which can be used by XPath
expressions.  Also added the ability to read RC files stored in the
user's home directory.
Added -x option

Added the -x option which will select nodes matching an XPath
expression.  Also added the notion of common starts and common ends for
pipelines.  This involves appending pipelines to other pipelines: which
is a little naive at the moment but works.  Also defaults to printing
the results to stdout.