Allow to disable tail commands from an extension

[esneider]

Allow extensions that do not work on files or buffers (like 'undo', 'dir', etc), to disable tail command parsing. More precisely, disable this behaviour:

End the string with a colon ':' followed by a Vim command to execute that
command after opening the file. If you need to use ':' literally, escape it
with a backslash: '\:'.

The extension will have to:

call add(g:ctrlp_ext_vars, {'sanstail': 0})