Allow .arclint to configure max line length of text linter

Test Plan:
Verified manually. Something like this in .arclint:

"linters" : {
  "text" : {
    "type" : "text",
    "include" : "(\\.(txt|py|html?)$)",
    "text.max-line-length": 200
  }

changes the line length. Something other than an integer there raises an error.

Reviewers: epriestley, Blessed Reviewers, Arcanist

Reviewed By: epriestley, Blessed Reviewers, Arcanist

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D9029