Add a user editable list of plugin names to skip

[Julix91]

Adds an input field:

When Plugin Name(s) is(/are) entered (separated by ;) then those plugins get skipped.

[Julix91]

Related to (but not quite the same as) https://github.com/wpengine/phpcompat/issues/67

[Julix91]

Updated it some more while actually using it. Found the input couldn't hold enough plugins easily. This doesn't break when using newlines as well, but it does seem to compress them on save.

(meaning that

Plugin 1;
Plugin 2;

is also perfectly valid.)

This is by far not the best solution yet - there should be a list of plugins with checkmarks for whether we want to include them or not - but that's more work than I could contribute today and I needed to get this task done. Hope that this is better than having no control over skipping any.