Document / Standardize parsing arbitrary data from queries into fields

[gtowey]

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.