Add flake8 option for maximum line length
Update changelog
Sort options alphabetically in syntax checker help
Add a function to find all declared checkers
Add tests for syntax checker description

Test for the appearance of the syntax checker docstring, executable,
config file and options in the syntax checker description.
Improve prompt when describing syntax checkers

[ci skip]
Fix commentary
Clarify sorting of `flycheck-declared-checkers`
Clarify the order of checkers in manual
Test that all syntax checkers are in the manual
Document option and config file variables
Fix markup
Move change log to manual
Add Flycheck as customization option