Add shellcheck as linter for bats files

doc/ale-bats.txt

@@ -0,0 +1,48 @@
+===============================================================================
+ALE Bats Integration                                        *ale-bats-options*
+
+
+===============================================================================
+shellcheck                                                  *ale-bats-shellcheck*
+
+g:ale_bats_shellcheck_executable               *g:ale_bats_shellcheck_executable*
+                                               *b:ale_bats_shellcheck_executable*
+  Type: |String|
+  Default: `'shellcheck'`
+
+  This variable sets executable used for shellcheck.
+
+
+g:ale_bats_shellcheck_options                     *g:ale_bats_shellcheck_options*
+                                                  *b:ale_bats_shellcheck_options*
+  Type: |String|
+  Default: `''`
+
+  With this variable we are able to pass extra arguments for shellcheck
+  for shellcheck invocation.
+
+  For example, if we want shellcheck to follow external sources (`see SC1091`)
+  we can set the variable as such:
+>
+  let g:ale_bats_shellcheck_options = '-x'
+<
+
+
+g:ale_bats_shellcheck_change_directory   *g:ale_bats_shellcheck_change_directory*
+                                         *b:ale_bats_shellcheck_change_directory*
+  Type: |Number|
+  Default: `1`
+
+  If set to `1`, ALE will switch to the directory the shell file being
+  checked with `shellcheck` is in before checking it. This helps `shellcheck`
+  determine the path to sourced files more easily. This option can be turned
+  off if you want to control the directory `shellcheck` is executed from
+  yourself.
+
+
+    autocmd BufEnter PKGBUILD,.env
+    \   let b:ale_bats_shellcheck_exclusions = 'SC2034,SC2154,SC2164'
+<
+
+===============================================================================
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:

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

@@ -35,6 +35,8 @@ Notes:
   * `shell` (-n flag)
   * `shellcheck`
   * `shfmt`
+* Bats
+  * `shellcheck`
 * BibTeX
   * `bibclean`
 * Bourne Shell