Use Travis CI for automated testing

[per1234]

Travis CI is used to run the following tests on the repository after every commit or pull request:

• Compile all example sketches for Mega, MKR Zero, Uno WiFi Rev2, Due using the latest version of arduino-cli.
• Use TravisBuddy to automatically comment on pull requests that result in a failed Travis CI build.
• Check all code files for compliance with the Arduino code formatting style.
• Check for commonly misspelled words.

Although it is not presently needed, I added an empty file: extras/codespell-ignore-words-list.txt. This file is to be used in case of false positives during the check for commonly misspelled words (all words in the file are ignored by codespell). My idea was that having the ignore system already in place would reduce the amount of hassle in the event of a false positive. If it is not considered desirable to add this file before it's needed, I'm happy to remove it from this PR.

I used the following files as a reference for this PR:

• https://github.com/arduino-libraries/WiFi101/blob/master/.travis.yml
• https://github.com/arduino-libraries/ArduinoIoTCloud/blob/master/.travis.yml

[per1234]

As a volunteer project, I prepared a demonstration of how I would configure Travis CI were this my own repository.

Example Travis CI build: https://travis-ci.org/per1234/SD/builds/531357867

.travis.yml: https://github.com/per1234/SD/blob/travis-ci-alternate/.travis.yml

Features:

• Compilation output is folded to make browsing the the log much faster and easier. A summary of each compilation is shown in the log that includes exit status, and warning count.
• Compile all sketches found in the examples folder, so it automatically adapts to new examples being added.
• Backwards compatibility testing using several "landmark" previous versions as well as the latest release of the Arduino IDE.
• Compilation summary report in CSV format is printed to the log as well as being published to a separate repository (for easier viewing and download). This report can be used to more easily compare changes in memory usage and warning count between revisions: https://github.com/per1234/CI-reports/blob/SD/build_00027/travis_ci_job_report_00027.001.tsv
• Check for common problems with library and example sketch structure.
• Check for common problems with library.properties
• Check for common problems with keywords.txt
• File formatting checks:
  • Non-Unix line endings
  • Trailing whitespace
  • Files starting with a blank line
  • Unnecessary true tabs
  • Extra blank lines at end of files

I thought this might provide some ideas for enhancements to Arduino's Travis CI configuration. I'd be happy to add any of the above individual features to this PR.

[per1234]

@lxrobotics I've added the requested build badge.

[aentinger]

@per1234 Great :+1: You are good to merge.