--js expression and {{js}} token

[75lb]

e.g.

$ renamer -f file -r 'file{{js}}' --js Date.now() * 
$ renamer -f file -r '{{js}}-file' --js process.cwd() * 
$ renamer -r '{{js}}.txt' --js sprintf("%10s", index) * 
#0000000001.txt
#0000000002.txt
# etc

$ renamer -r '{{index}} ({{js}}).txt' --js moment(new Date()).fromNow() * 
#1 (1 Day ago).txt
#2 (4 Months ago).txt

$ renamer -f '(\w+) -r '{{js}}' --js '$1.toTitleCase()' * 
# filename.txt -> Filename.txt