taco: add parser support for windowing/striding/index sets

[rohany]

Fixes #413.

This commit adds support for the command line tool to accept index
expressions containing windowing, striding and index sets. An example
of each of these features is added to the help message of taco:

taco "a(i) = b(i(1, 5))" -d=a:4          # Slice b[1:5]
taco "a(i) = b(i(1, 5, 2))" -d=a:2       # Slice b[1:5:2]
taco "a(i) = b(i({1, 3, 5, 7}))" -d=a:4  # Slice b[[1, 3, 5, 7]]