Add command for setting options, closes #38

The :VdebugOpt command allows you to get and set option values. It
provides autocomplete, so you no longer have to remember the names of
options.
Clarify meaning in docs regarding setting of options
Fix latest version number in HISTORY file

Why did no-one tell me? Damn you all...
Update HISTORY file for version 1.3.2
Update README to mention latest version and changes