query subcommand to retrieve metadata

[yannham]

Is your feature request related to a problem? Please describe.
Currently, there is no way to access the data contained in metavalues, in particular the documentation, which limits their usefulness.

Describe the solution you'd like
Add a query sub-command to the interpreter which shows the metadata associated to a value. One could provide a field path: nickel -f input.ncl query foo.bar which would be equivalent to evaluating let x = import "input.ncl" in x.foo.bar and look at the metadata. If x.foo.bar is a record without contract, it could also list all fields and associated metadata of this record. Long flag may specify which part of the meta value once is interested in, e.g. --doc, --contract, --default, etc.

The documentation could also be formatted following Markdown (using termad).

Example (this is illustrative, not necessarily the right output formatting):

$nickel query foo.bar <<< '{ foo = {bar | Str | doc "hello" | default 2}}'
foo.bar:
  - contract: Str
  - doc: hello
  - default value: Num(2)

$nickel query foo <<< '{ foo = {bar | Str | doc "hello" | default 2}}'
foo:
  * bar:
    - contract: Str
    - doc: hello
    - default value: Num(2)

$nickel query baz <<< '{ foo = {bar | Str | doc "hello" | default 2}; baz = "a"}'
No metadata found for field `baz`.

The syntax of the field path may also be more general, such as a term with a hole, which would allow stuff like _.$"^$foo%".baz. The future REPL should also have a similar command, but this is a separate issue.