add generic juttle value input control

[go-oleg]

Proposing creating a new input type called juttleValue that renders as a textarea and parses the entered value as a juttle value. This will allow users to enter anything from free-form Dates (:1 hour and 20 minutes ago:) to Objects ({ hello: "world"}) without us having to build a dedicated input type for every type of value Juttle supports.

Open Questions:

• Is juttleValue an okay name? Our convention is camelCase in juttle, but I think juttle_value looks a lot better.
• Assuming the parsing into values will happen on the client-side, we will want to bring the parser into the juttle-client-library. The other option would be to send the value as an unparsed string and have juttle-service/juttle understand the input type and parse the value.

cc @jadbox @demmer

[go-oleg]

Another thought, we can just call it value:

input a: value -default { hello: 'world' };

[dmajda]

value sounds good to me

[dmehra]

This depends on juttle/juttle#148