added shellcheck executable and options variable

updated docs

ale_linters/sh/shellcheck.vim

@@ -10,6 +10,16 @@ if !exists('g:ale_linters_sh_shellcheck_exclusions')
     let g:ale_linters_sh_shellcheck_exclusions = ''
 endif
 
+let g:ale_sh_shellcheck_executable =
+\   get(g:, 'ale_sh_shellcheck_executable', 'shellcheck')
+
+let g:ale_sh_shellcheck_options =
+\   get(g:, 'ale_sh_shellcheck_options', '')
+
+function! ale_linters#sh#shellcheck#GetExecutable(buffer) abort
+    return g:ale_sh_shellcheck_executable
+endfunction
+
 if g:ale_linters_sh_shellcheck_exclusions !=# ''
     let s:exclude_option = '-e ' .  g:ale_linters_sh_shellcheck_exclusions
 else
@@ -29,12 +39,14 @@ function! s:GetDialectArgument() abort
 endfunction
 
 function! ale_linters#sh#shellcheck#GetCommand(buffer) abort
-  return 'shellcheck ' . s:exclude_option . ' ' . s:GetDialectArgument() . ' -f gcc -'
+    return ale_linters#sh#shellcheck#GetExecutable(a:buffer)
+    \   . ' ' . g:ale_sh_shellcheck_options
+    \   . ' ' . s:exclude_option . ' ' . s:GetDialectArgument() . ' -f gcc -'
 endfunction
 
 call ale#linter#Define('sh', {
 \   'name': 'shellcheck',
-\   'executable': 'shellcheck',
+\   'executable_callback': 'ale_linters#sh#shellcheck#GetExecutable',
 \   'command_callback': 'ale_linters#sh#shellcheck#GetCommand',
 \   'callback': 'ale#handlers#HandleGCCFormat',
 \})

doc/ale-sh.txt

@@ -19,12 +19,35 @@ g:ale_linters_sh_shell_default_shell     *g:ale_linters_sh_shell_default_shell*
 -------------------------------------------------------------------------------
 shellcheck                                                  *ale-sh-shellcheck*
 
+g:ale_sh_shellcheck_executable                 *g:ale_sh_shellcheck_executable*
+
+  Type: |String|
+  Default: `'shellcheck'`
+
+  This variable sets executable used for shellcheck.
+
+
+g:ale_sh_shellcheck_options                       *g:ale_sh_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_sh_shellcheck_options = '-x'
+
+
 g:ale_linters_sh_shellcheck_exclusions *g:ale_linters_sh_shellcheck_exclusions*
 
   Type: |String|
   Default: `''`
 
   Set this variable to exclude test(s) for shellcheck (-e/--exclude option).
+  To exclude more than one option, separate them with commas.
 
 
 -------------------------------------------------------------------------------