Adding differences detection

[dmonllao]

Adds:

• Info about big differences when comparing runs using the web user interface, showing green and red cool colours
• New php CLI script that returns an exitcode according to the differences it finds

New settings to manage it:

• defaults to 4% for grouped and 2% single steps
• specify a whole json array with specifics for everything

[stronk7]

See comments, that also apply here @ https://github.com/moodlehq/moodle-performance-comparison/pull/17

[stronk7]

done, thanks!