Add grid_attrs argument to Model constructor

[spencerkclark]

Closes #182

[spencerahill]

Thanks for speeding this through @spencerkclark! I should have time on Friday to properly review this.

[spencerahill]

With the caveat that I'm a little rusty at thinking carefully about the codebase, I actually think the implementation looks good as is. A couple minor docs suggestions is all.

[spencerahill]

Awesome --- this looks ready to go @spencerkclark . Merge whenever you're ready, and thanks as always for the great work.

[spencerkclark]

@spencerahill one more minor update for you to have a look at; I added a reference table to the documentation showing the current built-in alternative names (I think it's a bit tricky for new users to find those at the moment). Let me know if that looks good and I'll go ahead and merge.

[spencerahill]

Great idea on the table. One tiny edit; go ahead and merge once that's done.

[spencerkclark]

Thanks for the review @spencerahill; merging now!