Support restricting mimetype in Asset fieldtype with "typefilter" option

[paulthewalton]

Related to issue #937.
Should be compatible with pull request #1288, but also may overlap some functionality

• passes "typefilter" option through to cp-assets component
• validates it as one of the pre-existing typefilters: "image", "video", "audio", "document", "archive", or "code"
• defaults to empty string (no filter)

Copied from my comment below:

Not sure what the browser compatibility targets are for this project, so I've tried to keep it as wide as possible.

Usage:

In Asset fieldtype options:

{
"typefilter": "image"
}
Setting this option will function the same as picking the typefilter from the dropdown in the UI, but the user will be unable to change the filter. Anything other than one of the predefined typefilter names (case-insensitive) will be discarded, and filtering will work as usual.

[aheinze]

Thanks for the contribution and initializing action to provide a solution for it. Fixed in e281ffe