ncm-metaconfig: add support for a preamble

Add support for including a preamble at the start of a file written by
ncm-metaconfig. This can be used to include contextual information in
the form of a comment since none of the modules supported by
ncm-metaconfig allow the insertion of comments into the files they
generate.
Merge pull request #62 from ned21/metaconfig-preamble

ncm-metaconfig: add support for a preamble