add optional filter to select field with contenttype values

[rossriley]

Tiny little feature that allows you to specify an optional filter when the values for a select field are pulled from a contenttype.. eg:

relatednews:
    type: select
    multiple: true
    values: pages/title
    filter: {categories:news}
    autocomplete: true

[bobdenotter]

Amazeballs!

I've added an issue to add some docs for this: bolt/docs#192