* 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* Remove outdated comments about vcol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
