Document / Standardize parsing arbitrary data from queries into fields

Currently it would be helpful to be able to easily add more custom fields besides the current hostname and size. For instance, you could add a query comment that includes the originating application user, or name of the application feature making the query, then be able to correlate query performance to application-specific categories. This is quite helpful for most end-users.

There are several parts that need to happen:
1. fields and values need to be parsed from the query in the --filter argument in pt-query-digest
2. Those columns must be added to the history table as fieldname_max
3. The unique index on the history table should include those columns.

This should first just be documented, showing people how they can do this for themselves.