add rebar_string:trim function for removing all leading/trailing whitespace
add rebar_string:trim function for removing all leading/trailing whitespace

This function was used for trimming the version from a `{file, File}` app
vsn in .app.src but caused a crash because it did not exist.

rebar/rebar3,vladdu/rebar3,lrascao/rebar3,saleyn/rebar3,erlang/rebar3,tsloughter/rebar3,rebar/rebar3