Support for per-field limits

[JamesSkemp]

If adding a counter to multiple fields, with each having a different maxlength, the limit on the last counter on the page will be used for all other counters. This stores the limit on the respective counter instead.

[JamesSkemp]

This is my first attempt at a pull request, so please be gentle :)

This is related to #10.

While it seemed to work, once I started typing into a field I found that the maximum length had been set to whatever was set on the last field on the page (options.limit is updated for each field, with the last having the final say).

This stores the length on the counter element itself instead, which is then checked.