Ready more linters and linter functions for .arclint

Summary:
Ref T3186. Ref T2039. Continues work on readying linters for `.arclint`.

  - **Ruby**: Make this an ExternalLinter.
  - **Priority**: Currently, linters have an implicit "correct" order (notably, the "NoLint" linter needs to run before other linters). Make this explicit by introducing `getLinterPriority()`.
  - **Binaries**: Currently, linters manually reject binary files. Instead, reject binary files by default (linters can override this if they do want to lint binary files).
  - **Deleted Files**: Currently, linters manually reject deleted files (usually in engines). Instead, reject deleted files by default (linters can override this).
  - **Severity**: Move this `.arclint` config option up to top level.
  - **willLintPaths()**: This method is abstract, but almost all linters provide a trivial implementation. Provide a trivial implementation in the base class.
  - **getLintSeverityMap()/getLintNameMap()**: A bunch of linters have empty implementations; these are redundant. Remove them.
  - **Spelling**: clean up some dead / test-only / unconventional code.
  - **`.arclint`**: Allow the filename, generated, nolint, text, spelling and ruby linters to be configured via `.arclint`.

Test Plan:
https://github.com/epriestley/arclint-examples/commit/458beca3d65b64d52ed612904ae66eb837118b94

Ran unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: Firehed, aran

Maniphest Tasks: T2039, T3186

Differential Revision: https://secure.phabricator.com/D6805