#307 hfst-fst2fst should warn if given both -w and -O

[Kevin Brubeck Unhammer]

I thought -w was weighted and -O was optimised, so I tried

$ hfst-fst2fst -w -O -i sme.automorf.hfst  -o foo.hfst

but got an optimised unweighted hfst out, since -O means "optimised unweighted", not just "optimised".

Maybe there should be a warning if the user passes conflicting arguments, instead of silently overriding previous arguments.