Add document identifier and committee for MEP and "Brochure Appendix" documents

[ronaldtse]

From @Intelligent2013:

@opoudjis in source XML this data are absent:

to produce a cover page. Could you add it please?

Originally posted by @Intelligent2013 in #5 (comment)

[opoudjis]

I've already done this in response to #5 (comment). The document identifiers are BIPM SI MEP __1 (and in one case __2), where the __ is populated by a title case version of the unit discussed (so S for s, Kg for kg).

The full name of the committee has been differentiated from the abbreviation; both are now document attributes.