Issue #316: Add additional formatters for multi-value fields.

[jenlampton]

Not ready yet!

Questions:

1. What should we do with the list field? -- can we use these for single-value displays?
2. Where should we put the generic field functions? Right now they are in field.module, but I think maybe they should go into list module?

Has support for:

• Text
• Link (configure form + summary still missing)
• Number (configure form + summary still missing)

Needs:

• Limit to only multi-value fields?
• Taxonomy term support
• Entity reference support

Addresses backdrop/backdrop-issues#316

[backdrop-ci]

Related to: backdrop/backdrop-issues#316