Add size option to select and select-advanced field types.

Default to five items for multi-selects, and 0 for single-selects. According to https://developer.mozilla.org/en-US/docs/HTML/Element/select the default size for single-selects should be 0 and not 1.
Changes to select to allow a default option

Seperated the option creation to a seperate function, added a `default`
argument to create a default option
New posts field

New `posts` field takes 3 args: `post_type`, which defaults to `post`,
`field_type`, which currently supports `select` and `select_advanced`
and '`query_args`, to better control the query that pulls the posts up
Tweaking to image and file fields