Support offsets on the right hand side

[ahwillia]

As per the feature request here: #6

I have a prototype working but it needs much more extensive testing. @ntezak - can you do me a favor and try checking out this branch to see if this meets your needs? Are the conventions right?

• I only support adding/subtracting an offset
• Offsets have to be integers
• This doesn't work yet: This also works now:

offset = 5
@einsum A[i] = B[i+:offset]

Checklist for merging:

• Fully document conventions (e.g. we don't always enforce equality on bounds checks now.)
• Clean up the tests -- I'd like to use v0.5 testing but BaseTestNext is slow. I may just wrap every set of tests in a let statement for now.
• @einsum A[i] = B[i+:offset]

[ahwillia]

I'm happy with this - merging now. Will clean up documentation before tagging next version.