Add auto subscriptions via module

[surik]

This commit introduces feature for making subscription automatically for each new entry. By default it is off. If you want to use auto subscription you should turn it on in reporter options:

{exometer,
    {reporters, [
        {exometer_report_influxdb, [
            {autosubscribe, true},
            {subscriptions_module, exometer_influxdb_subscribe_mod},
            {protocol, http},
            {host, <<"localhost">>},
            {port, 8086},
            {db, <<"exometer">>},
            {tags, [{region, ru}]}
        ]}
    ]}

Module may look like:

-module(exometer_influxdb_subscribe_mod).
-compile([export_all]).

subscribe([metric, test], histogram) ->
    Tags = [{tags, [{type, {from_name, 2}}]}],
    [{[metric, test], min, 1000, Tags},
     {[metric, test], max, 1000, Tags},
     {[metric, test], median, 1000, Tags}];
subscribe([metric, test1], histogram) ->
    Tags = [{tags, [{type, {from_name, 2}}]}],
    [{[metric, test1], max, 1000, Tags},
     {[metric, test1], median, 1000, Tags}];
subscribe(_, _) -> [].

subscribe/2 calls for each new entry and it should return list or just one subscription.

Here subscription is:

 {exometer_report:metric(),
  exometer_report:datapoint(),
  exometer_report:interval(),
  exometer_report:extra()}