Add proselint checker

[marsam]

Hi:

This is a rebase of #939

Minor changes with respect to #939:

• Use standard input
• Added gfm-mode to :modes
• Replaced json-read-from-string with flycheck-parse-json

Related:

• Add new recipe for flycheck-proselint. melpa/melpa#4938

[purcell]

Looks good to me.

[purcell]

I've looked through this, and it looks like a no-brainer, but would a more active maintainer care to click "merge" if they agree please?

[cpitclaudel]

@purcell Thanks for the review. @marsam Thanks for your patience and your excellent work!

[cpitclaudel]

Any reason to use (pcase (`"error" …) …) rather than (pcase ("error" …) …) btw? We use the former in multiple places (and it's valid according to the docs), but I hadn't paid attention to it before and I always use the latter.

[fmdkdd]

@cpitclaudel I never quite managed to understand the docstring of pcase on that point to figure out if they were really equivalent, so I always go with the backtick, for coherency with the existing code.

[cpitclaudel]

They expand to the same thing :) (as an example, here's what our use of pcase in the checkstyle parser expands to:)

(cond ((equal .severity '"error")
       'error)
      ((equal .severity '"warning")
       'warning)
      ((equal .severity '"info")
       'info)
      (t 'error))

[fmdkdd]

@cpitclaudel Good to know (but still confusing to have two ways to state the same thing) :)