[LINTER] Add a markdown linter

Summary:
This linter is fairly limited for now, and will only check for dead
local links (links within the project scope).

Test Plan:

arc lint --everything

Revert one of the markdown changes from this diff:

arc lint

Check the linter raises an error.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7411