Add VIM file type plugins for Apache and XML

vim/ftplugin/apache.vim

+" Only set the b:ml_commentstring if it hasn't already been set
+if !exists("b:ml_commentstring")
+    let b:ml_commentstring = '# %s'
+end

vim/ftplugin/xml.vim

+" Only set the b:ml_commentstring if it hasn't already been set
+if !exists("b:ml_commentstring")
+    " Add whitespace to the modeline commentstring
+    let b:ml_commentstring = '<!-- %s -->'
+end