Checks overhaul

[gdude2002]

As it stands, checks are pretty simple and there's a lot of power in their simplicity. However, there are some situations where checks are not enough, and the user needs to have a message returned to them.

This issue proposes a new style of checks - very similar to the current style, they would simply take an event and return a value. However, that value should be a CheckResult rather than a boolean - this would allow check developers to specify a message or translation key to be returned to the user, if the check fails.

This should be configurable on a per-command and per-extension basis as well.

[gdude2002]

I decided to take a different approach to this, as CheckResult was cumbersome and a lot of extra context had to be passed around. Instead, checks now make use of a CheckContext type deal, with pass and fail functions available.

This is in 1.4.3-SNAPSHOT, and it'd be great if people would try it.

[gdude2002]

This is now in a release version, so I'll consider it good. Feel free to open another issue if there are problems.