Allow : for assignment inside inline tables

[lizelive]

hello = { world : 42 }

should be equivalent to

hello = { world = 42 }

this synergies with #516 and #902 to make all json objects that don't include null valid toml.

pros:

1. paste json
2. easier to move over to toml
3. more choice
4. more readable

[pradyunsg]

make all json objects that don't include null valid toml.

This isn't a goal.