User needs to adjust $extra_options for mysqld_exporter 0.11 and newer

[bastelfreak]

I needed a quick way to get this information into the CHANGELOG.md. This issue relates to #247. It is now supported by this module, that users install mysqld_exporter v0.11.0 or newer by setting:

---
prometheus::mysqld_exporter::version: '0.11.0'

The format of additional arguments changed from double dashes to single dashes. Users that provide their own arguments in $extra_options need to adjust from - to --.