Limit the amount of printed warnings/errors

[Awjin]

In #1319, an overwhelming number of deprecation warnings were printed to the terminal, forcing the user to end the process.

We should add a ceiling to the number of warnings that get printed to the terminal—100?—that, when surpassed, just prints the remaining count. Something along the lines of:

Warning: This is warning #1.

Warning: This is warning #2.

...

Warning: This is warning #${ceiling}.

${remaining count} more warnings.

[stof]

Rendering the remaining count will be tricky: warnings are rendered during the compilation, not at the end.

For errors, it is impossible to have multiple ones: reaching an error means the end of the compilation.

[Awjin]

We will also need a way to explicitly enable printing all warnings (e.g. with a --verbose flag).

[tobiasweibel]

Is there an option to hide all the deprecation warnings for version 1.*? In one of my projects we do have a dependency to bootstrap-sass which probably won't get fixed.