add doc and test

doc/ale-fish.txt

@@ -10,5 +10,22 @@ displaying errors if an error message is not found.
 If ALE is not showing any errors but your file does not run as expected, run
 `fish -n <file.fish>` from the command line.
 
+===============================================================================
+fish_indent                                               *ale-fish-fishindent*
+
+g:ale_fish_fishindent_executable             *g:ale_fish_fishindent_executable*
+                                             *b:ale_fish_fishindent_executable*
+  Type: |String|
+  Default: `'fish_indent'`
+
+  This variable can be changed to use a different executable for fish_indent.
+
+g:ale_fish_fishindent_options                   *g:ale_fish_fishindent_options*
+                                                *b:ale_fish_fishindent_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be set to pass additional options to fish_indent.
+
 ===============================================================================
   vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

doc/ale-supported-languages-and-tools.txt

@@ -144,6 +144,7 @@ Notes:
   * `SyntaxErl`
 * Fish
   * `fish` (-n flag)
+  * `fish_indent`
 * Fortran
   * `gcc`
   * `language_server`

doc/ale.txt

@@ -2594,6 +2594,7 @@ documented in additional help files.
   eruby...................................|ale-eruby-options|
     ruumba................................|ale-eruby-ruumba|
   fish....................................|ale-fish-options|
+    fish_indent...........................|ale-fish-fishindent|
   fortran.................................|ale-fortran-options|
     gcc...................................|ale-fortran-gcc|
     language_server.......................|ale-fortran-language-server|