Add commands for jumping through problems in all buffers

ALE offers some commands for jumping through errors within a single Vim buffer, such as `:ALENext` and friends. Vim offers `:cnext` and `:cprev` for jumping through errors across all buffers, using the quickfix list. `:cnext` and `:cprev` don't work with ALE, because ALE often resets the quickfix list, which resets the position in the quickfix window.

Some new commands could be implemented for jumping through errors across all buffers. The process would work like so:

1. Invoke a command like `:ALENextGlobal` or whatever the command ends up being called to jump to an error.
2. Combine and de-duplicate the lists from all buffers, sorting the lists by the absolute paths to the files.
3. Find the relative position in the list with a tuple `(filename, line, column)` and jump to the position in that file.