Close #2556 - Support filename mapping

ALE now supports mapping files between different systems for running
linters and fixers with Docker, in virtual machines, in servers, etc.

autoload/ale/filename_mapping.vim

@@ -0,0 +1,22 @@
+" Author: w0rp <devw0rp@gmail.com>
+" Description: Logic for handling mappings between files
+
+" Invert filesystem mappings so they can be mapped in reverse.
+function! ale#filename_mapping#Invert(filename_mappings) abort
+    return map(copy(a:filename_mappings), '[v:val[1], v:val[0]]')
+endfunction
+
+" Given a filename and some filename_mappings, map a filename.
+function! ale#filename_mapping#Map(filename, filename_mappings) abort
+    let l:simplified_filename = ale#path#Simplify(a:filename)
+
+    for [l:mapping_from, l:mapping_to] in a:filename_mappings
+        let l:mapping_from = ale#path#Simplify(l:mapping_from)
+
+        if l:simplified_filename[:len(l:mapping_from) - 1] is# l:mapping_from
+            return l:mapping_to . l:simplified_filename[len(l:mapping_from):]
+        endif
+    endfor
+
+    return a:filename
+endfunction