Don't rename files and use first_line as filename for OpenPad

Kudos on a great piece of software

Two patches

1. Add an option g:pad_rename_files to disable renaming of files when the title line changes. The default value is *1*, i.e. by default it keeps the current behaviour. If g:pad_rename_files is 0, then pad_local.update() will not rename the file.
2. When OpenPad gets a first_line argument, the timestamp is still being used as the file name, this patch fixes that

The rationale for this is to keep the note paths static (even if they are a timestamp). I use other apps to open my markdown notes and sometimes run into issues when some older paths are no longer valid because I changed the title line.