Allow to attach a user-defined tag as meta-info

[danielmitterdorfer]

It's sometimes useful to attach user-defined info to each metrics record, like what's the intention of the benchmark. Therefore we should allow users to specify a tag on the command line which will then be attached to each metrics record. In order to avoid clashes with internal meta data, we always prepend the user-specified tag with "tag_".

Intended command line interface:

esrally --user-tag=intention:baseline-ticket-12345

where tag_intention is the meta info key, and baseline-ticket-12345 the meta info value. It will end up as cluster-level metric in the meta info structure of a metrics record (see also #54).