Ion parsing bug when dealing with string expansion

Created by: stratact

Reproduction: echo $(let free=[@(free -h)]; echo @free[6]@free[8]/@free[7]).

Expected behavior: Should output Mem:4Gi/32Gi.

Actual behavior: Outputs ]; echo /) instead.

Build information: `rustc 1.28.0-dev (a2c4d4e2f 2018-05-28)

ion 1.0.0-alpha (x86_64-unknown-linux-gnu) rev e3bd8ce6

Blocking/related: blocks me from doing the Memory usage option for Ion's prompt builtin command: #754