v.1.7.0

points of update

• implemented join and dropjoin function.

### join F[2], F[3], and F[4] with @ ###
$ echo {a..e} | ./opy -o @ '[join(F,2,4)]'
b@c@d
### drop the second field ###
$ echo {a..e} | ./opy '[dropjoin(F,2)]'
a c d e
### drop the second and third field ###
$ echo {a..e} | ./opy '[dropjoin(F,[2,3])]'
a d e
### F0 is also accepted. ###
$ echo {a..e} | ./opy '[dropjoin(F0,[2,3])]'
a d e

• added comments