Add hooks to check .sh and .bash headers - Fixes #1491 (#1765)

* Add hooks to check .sh and .bash headers
* Adds hooks/ to clean_files.txt

themes/90210/90210.theme.bash

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+# shellcheck shell=bash
 SCM_THEME_PROMPT_DIRTY=" ${red}✗"
 SCM_THEME_PROMPT_CLEAN=" ${bold_green}✓"
 SCM_THEME_PROMPT_PREFIX=" |"

themes/agnoster/agnoster.theme.bash

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+# shellcheck shell=bash
 # vim: ft=bash ts=2 sw=2 sts=2
 #
 # agnoster's Theme - https://gist.github.com/3712874

themes/barbuk/barbuk.theme.bash

@@ -1,5 +1,4 @@
-#!/usr/bin/env bash
-# shellcheck disable=2034,2154
+# shellcheck shell=bash
 
 # Theme custom glyphs
 SCM_GIT_CHAR_GITLAB=${BARBUK_GITLAB_CHAR:='  '}

themes/powerline/powerline.base.bash

@@ -1,3 +1,4 @@
+# shellcheck shell=bash
 # Define this here so it can be used by all of the Powerline themes
 THEME_CHECK_SUDO=${THEME_CHECK_SUDO:=true}
 

themes/powerline/powerline.theme.bash

@@ -1,4 +1,4 @@
-#!/usr/bin/env bash
+# shellcheck shell=bash
 
 # shellcheck source=../../themes/powerline/powerline.base.bash
 . "$BASH_IT/themes/powerline/powerline.base.bash"