feat: Default to full path to main ledger file, allows changing directories

Closes #150

Author: alerque

autoload/ledger.vim

@@ -33,7 +33,7 @@ else
 endif
 
 if !exists('g:ledger_main')
-  let g:ledger_main = '%'
+  let g:ledger_main = '%:p'
 endif
 
 if !exists('g:ledger_dangerous_formatprg')

doc/ledger.txt

@@ -388,7 +388,7 @@ behaviour of the ledger filetype.
 
 * The file to be used to generate reports:
 
-        let g:ledger_main = '%'
+        let g:ledger_main = '%:p'
 
   The default is to use the current file.