[ADD][VIM] Shellcheck options for ALE

I want shellcheck to detect which shell is most appropriate on the `sh`
filetype, but explicitly use the `bash` dialect on `bash` and `zsh`
filetypes.

I also really don't like nullary conditions in shell, so I'll screen for
those too.

I might turn on the 'quote-safe-variables' check too, because I think
quoting variables is a good habit to take.

vim/.vim/ftplugin/bash.vim

@@ -1,2 +1,8 @@
 " Use shfmt as ALE fixer for bash
 let b:ale_fixers=[ 'shfmt' ]
+
+" Indent with 4 spaces, simplify script, indent switch cases, use Bash variant
+let b:ale_sh_shfmt_options='-i 4 -s -ci -ln bash'
+
+" Use bash dialect explicitly, require explicit empty string test
+let b:ale_sh_shellcheck_options='-s bash -o avoid-nullary-conditions'

vim/.vim/ftplugin/sh.vim

@@ -1,2 +1,8 @@
 " Use shfmt as ALE fixer for sh
 let b:ale_fixers=[ 'shfmt' ]
+
+" Indent with 4 spaces, simplify the code, indent switch cases, use POSIX
+let b:ale_sh_shfmt_options='-i 4 -s -ci -ln posix'
+
+" Require explicit empty string test
+let b:ale_sh_shellcheck_options='-o avoid-nullary-conditions'

vim/.vim/ftplugin/zsh.vim

@@ -1,2 +1,8 @@
 " Use shfmt as ALE fixer for zsh
 let b:ale_fixers=[ 'shfmt' ]
+
+" Indent with 4 spaces, simplify script, indent switch cases, use Bash variant
+let b:ale_sh_shfmt_options='-i 4 -s -ci -ln bash'
+
+" Use bash dialect explicitly, require explicit empty string test
+let b:ale_sh_shellcheck_options='-s bash -o avoid-nullary-conditions'