Add :enabled property to syntax checkers

[swsnr]

Add a new :enabled FUNCTION property to syntax checkers which behaves like :predicate except that it's only called the first time a syntax checker is to be used in a buffer. If the function returns a non-nil value the checker is put in a whitelist (i.e. a buffer-local variable) to prevent further invocations of :enabled, and if it returns nil it's put into flycheck-disabled-checkers to prevent any further use of it.

This function would allow for more expensive checks whether a syntax checker is available.

Follow up of a Gitter discussion and #1085.

[swsnr]

@Simplify I've taken the liberty to assign this issue to you but please feel free to ping me if you don't get to implement it. I can add it as well tomorrow evening—though, if you'd take it, I'd spend the evening on #1072 😎

[Simplify]

yes, I'll take this one, great opportunity to dig dipper into flycheck.

[Simplify]

flycheck-may-use-checker (checker) is executed every time or only once for every buffer? If only once we can skip maintaining white-listed buffer-list for every checker.

[swsnr]

@Simplify It runs every time.