Add validation messages for `minChars` and `maxChars` (Fixes #24)

* Adds a new configuration option for `maxChars`. Works similarly to `minChars`
* Added validation message for `minChars`
* Added validation message for `maxChars`
* Added `onRenderErrorMessage` callback which can be overridden in order to
  customize validation behavior.
* Added corresponding tests.
* Updated readme and change log.
* Bumped version.