Allow array slicing

[Soares]

Not a big priority, but it would be pretty cool to have python-style array slicing. T'would be a bit different due to the 1-indexing of arrays, but I envision something like this:

> set x a b c d
> echo $x[2:3]
b c
> echo $x[2:]
b c d
> echo $x[:2]
a

[ridiculousfish]

We have this thanks to maxfl's awesome contribution at #212

> set x a b c d
> echo $x[2..3]
b c

It doesn't allow omitting indexes, but you can use -1 to index from the end.

> set x a b c d
> echo $x[2..-1]
b c d

also note that ranges are inclusive, which is a little weird, but makes sense when you consider that fish indexes starting from 1. So

> echo $x[1..(count $x)]
a b c d

ought to output everything.

[Soares]

Sweeeet! Should have read the manual harder before posting, my apologies.