Support dynamic specification of reporter type map info

When adding a subscription dynamically, there hasn't been support
for updating the type maps in the reporters. Two alternatives have
been added to enable dynamic management of type mapping.

1. Add {report_type, Type} in Extra when adding the subscription.
   This information will override whatever information is in the
   type map.

2. Wildcard type maps
   It's now possible to use a wildcard pattern in a type_map entry.
   E.g. {[my_app, counters, '_'|'_'], counter}.
   The function exometer_util:report_type(Key, Extra, TypeMap)
   searches the type map from the top down, until it finds a
   pattern that matches the key.
allow report subscriptions with list of dps
Merge branch 'master' into uw-report-multiple-datapoints
documentation + minor fixes in exometer_report
Merge branch 'uw-report-multiple-datapoints' of github.com:Feuerlabs/exometer into uw-report-multiple-datapoints
Merge pull request #27 from Feuerlabs/uw-report-multiple-datapoints

Uw report multiple datapoints