Allow to disable tail commands from an extension

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})
```