Use buffer variables for ALE, and load Vue linters

[w0rp]

Someone wants to add vls support over here, and for that 'vue' will need to be included in the aliases for vue files. I switched the variables to buffer-local variables, so the ftplugin file doesn't mess with global variables.

I removed the exists check because the buffer variables are harmless, and ALE could be loaded after the file is loaded.

[adriaanzon]

Thanks!