More granular configuration of when to check encoding

When opening an existing file (typically when browsing a gem via `bundle open`), this plugin would change the files right as I opened the file. I didn’t like that.

So I changed the plugin so you can configure when it should check the encoding:

* `check_encoding_on_load`
* `check_encoding_on_keystroke`
* `check_encoding_on_save`

Note that this breaks compatibility with the existing `checking_encoding_on_pre_save_only` configuration option. If wanted, it should be possible to add check for that old option along with the new ones.

I saw no way to keep the old configuration option, as its name ends with “_only”.