rename columns name using a function

[femtotrader]

Following #20 (comment)

A rename! function implementation could also accept a function as 2nd parameter which could rename column name in place.

Something like

rename!(data, c->replace(c, " ", ""))

Although TS column names are Symbol... not String so it will become

rename!(data, c->Symbol(replace(String(c), " ", "")))

but I'd prefer the Symbol/String complexity be hidden.