Update 'include' pattern.

Use of '!' prepending Ledger command directives has been deprecated (see
Ledger's manual, §4.7.2).

This commit makes '!' optional in the 'include' pattern.

Author: Lifepillar

ftplugin/ledger.vim

@@ -15,7 +15,7 @@ let b:undo_ftplugin = "setlocal ".
 
 setl foldtext=LedgerFoldText()
 setl foldmethod=syntax
-setl include=^!include
+setl include=^!\\?include
 setl comments=b:;
 setl commentstring=;%s
 setl omnifunc=LedgerComplete