The statusbar now keeps its state in a separate variable, in order to
avoid excess iterations. The engine now updates said variable on run,
and a new function is made available for external statusbars to call (to
avoid dependencies on internal implementation details of ale).
To keep things light, the status bar code is not loaded unless invoked
by the user or an external plugin. On the first load it will update
itself from the global loclist, after that, the engine will handle all
updates.
The external integration function, `ale#statusline#Count()`, will return
a tuple in the format [E, W] (where E is errors, W is warnings), unless
no data exists (ie, the plugin doesn't have a linter for a file or has
not run yet), in which case it returns 0/false.
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