FR: in dcast, with g ~ ., don't put . in the col names

[franknarf1]

Looking at this SO question: http://stackoverflow.com/q/30759367/

dcast(setDT(melt(trees)), variable ~ ., fun=list(sd, mean))

# No id variables; using all as measure variables
#    variable value_sd_. value_mean_.
#1:    Girth   3.138139     13.24839
#2:   Height   6.371813     76.00000
#3:   Volume  16.437846     30.17097

It would be nice to have names value_sd and value_mean without the _. at the end. Related to #1810

Maybe it would be better not to handle this piecemeal (as my two FRs have suggested), but instead with some sort of customizable regex argument name.patt = "\\meas_\\fun_\\rhs".