Add initial support for settings to overwrite ale shell

doc/ale.txt

@@ -1984,6 +1984,24 @@ g:ale_windows_node_executable_path         *g:ale_windows_node_executable_path*
   scripts are executed with whatever executable is configured with this
   setting.
 
+g:ale_shell                                                      *g:ale_shell*
+
+  Type: |String|
+  Default: `'&shell'`
+
+  This variable is used to determine which shell ale will use to execute 
+  commands. This variables defaults to the value of the vim option '&shell'
+  which corresponds to the $SHELL environment variable. For example 
+  if `$SHELL == '/bin/bash'`, but you want to use zsh, set `g:ale_shell = '/bin/zsh'.`
+
+g:ale_shell_arguments                                  *g:ale_shell_arguments*
+
+  Type: |String|
+  Default: `'&shellcmdflag'`
+
+  This variable is used to determine what commands vim will pass to the shell
+  to execute it's commands. By default, `&shellcmdflag` would be set to the
+  value of '`-c'`.
 
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 6.1. Highlights                                                *ale-highlights*