Print job statistics

[jbrazio]

Fixes #1971

This PR introduces Print Statistics, by default they are disabled, to turn them on the user must #define PRINTCOUNTER in Configuration.h.

//
// Print Counter
//
// When enabled Marlin will keep track of some print statistical data such as:
//  - Total print jobs
//  - Total successful print jobs
//  - Total failed print jobs
//  - Total time printing
//
// This information can be viewed by the M78 command.

A sample output of the M78 command:

M78
Print statistics: Total: 3, Finished: 2, Failed: 1, Total print time: 3h 2min
ok

Note: Still pending the update of the example configuration files.

[thinkyhead]

That's a pretty exemplary PR — and feature. Classy, even.

A documentation note: We haven't written the definitive documentation for Version 1.1.0 yet, so I feel it's ok to have another feature in it, so long as it is disabled by default. Same goes for configuration options (if we can make them simpler). But we are only a week perhaps from release, so nothing too disruptive like a new probe type today!

Don't forget to add a line in the header of Marlin_main.cpp that documents M78 with the others.

[jbrazio]

Actually I designed this in a way that for the end-user the only configuration option he should access is PRINTCOUNTER, the other two which control the time between tick() and save() cycles shouldn't be user editable thus I have them as constants inside the object. If you feel this should change and give more freedom to the user to define those two flags I can expose them on Configuration.h.

[thinkyhead]

"successful"

[jbrazio]

It's fixed with e34f465.

[thinkyhead]

I don't see the need for any major changes or to make it extra configurable.

[jbrazio]

@thinkyhead it should be ready to be merged now.