Error parsing $++a

[benhoyt]

Possibly related to #79, but opening a separate issue for visibility: GoAWK errors when parsing $++a. Compare:

$ echo -e '1 2 3\na b c\nd e f' | gawk '{ print $++a }'
1
b
f
$ echo -e '1 2 3\na b c\nd e f' | goawk '{ print $++a }'
<cmdline>:1:10: expected expression instead of ++
{ print $++a }
         ^