Working directories are now set seperately from the commands so they
can later be swapped out when running linters over projects is
supported, and also better support filename mapping for running linters
on other machines in future.
* FIX: use mix from the project root directory
* Move find root project function to autoloaded handlers
* add tests for #ale#handlers#elixr#FindMixProjectRoot
* Remove 'col' from linters where it is hardcoded to 1
When 'col' is 1, the first column will get highlighted for no reason. It
should be 0 (which is the default).
In the scalac linter there was also a check about the outcome of
`stridx`. It would set l:col to 0 if it was -1, and then it uses
`'col': l:col + 1` to convert the outcome of `stridx` to the actual
column number. This will make 'col' equals 1 when there is no match. We
can remove the check because `-1 + 1 = 0`.
* Remove outdated comments about vcol
vcol was added as a default, and the loclists that follow these comments
do not contain 'vcol' anymore
