Fdiff

- Bump fonttools from 4.2.2 to 4.2.4
- Bump multidict from 4.7.3 to 4.7.4
- Bump pycares from 3.1.0 to 3.1.1

- `fdiff` executable: added support for external executable tool diff execution with a new `--external` option
- Library: major refactor of the `fdiff.diff` module
- Library: add new public `external_diff` function to the `fdiff.diff` module
- Library: minor refactor of `fdiff.color` module (removed unnecessary import)
- [bugfix] Library: fixed bug in ANSI color output
- updated fontTools dependency to v4.2.2
- updated aiohttp dependency to v3.6.2
- pinned the versions of the following dependencies of dependencies in the requirements.txt file: async-timeout, attrs, cffi, chardet, idna, multidict, pycares, pycparser, yarl

- `fdiff` executable: Fix help message - added information about pre/post file argument support for URL in addition to local file paths

- Performance optimizations - Library: New default parallel TTX XML dump on systems that have more than one CPU, falls back to sequential execution on single CPU systems - `fdiff` executable: New `--nomp` option that overrides the default multi processor optimizations
- `fdiff` executable: Added new default standard output user notification that no difference was identified when the files under evaluation are the same. This replaces no output in the standard output stream and an exit status code of zero as the indicators that there were no differences identified between the files.

- Added support for remote font files with asynchronous I/O GET requests. This feature supports combinations of local and remote font file comparisons.
- `fdiff` executable: added support for remote font files with command line URL arguments
- `fdiff` executable: refactored unified diff error message formatting
- Library: add new `fdiff.remote` module
- Library: add new `fdiff.aio` module
- Library: add new `fdiff.exceptions` module
- Library: refactored `fdiff.diff.unified_diff()` function to support remote files through URL
- Library: refactored local file path checks to support remote files via URL
- added new aiohttp, aiodns, aiofiles dependencies to requirements.txt
- added new aiohttp, aiodns, aiofiles dependencies to setup.py
- added pytest-asyncio dependency to setup.py [dev] install target
- added pytest-asyncio dependency instatllation to tox.ini, .travis.yml, .appveyor.yml configuration files
- Py3.6+ updates: removed ` -*- coding: utf-8 -*-` header definitions (Thanks Niko!)
- updated fontTools dependency to v4.0.1 (from v4.0.0)
- Updated README.md documentation

- Added support for head and tail diff output filter functionality - `fdiff` executable: add support for filtered diff output by top n lines with new `--head` option - `fdiff` executable: add support for filtered diff output by last n lines with new `--tail` option - Library: add new `fdiff.textiter` module
- Add README.md table of contents