Add test for missing or extra keywords in keywords.txt

[per1234]

Parse library source files to generate a list of keywords and then compare these to the contents of keywords.txt.

This bash script: https://github.com/markszabo/IRremoteESP8266/blob/master/tools/mkkeywords looks like it might be useful.

Other keywords.txt generators:

• https://github.com/sparkfun/Keyword_File_Generator
  • I tried it and it won't work:
    • paths must use Windows separator: https://github.com/sparkfun/Keyword_File_Generator/blob/e4c064508ea2ca17bfdaa3207f3187b5c8fa0786/keywords_populator.py#L6
    • Any line that contains "class" is expected to supply a class name as the second word: https://github.com/sparkfun/Keyword_File_Generator/blob/e4c064508ea2ca17bfdaa3207f3187b5c8fa0786/keywords_populator.py#L120
      • This breaks the script whenever the word "class" is used not in a class definition (e.g., a comment), or in the case of an enum class
• https://github.com/bengtmartensson/KeywordsTxtGenerator
• https://github.com/r89m/arduino-keywords
• https://gist.github.com/don/5748655
• https://github.com/Harryman/Arduino-header-keyword-creator
• https://github.com/aster94/Utilities/blob/master/extras/keywords.py
• update keywords tzapu/WiFiManager#451 is relevant and more information might eventually appear there.