Add mat"" custom string literal

[simonster]

The @matlab macro currently takes Julia syntax and tries to convert it to MATLAB syntax. This mostly works because the syntax is similar, but it is occasionally confusing and/or broken (see #26). This is an alternative approach that is hopefully both easier to use and harder to break.

The mat string literal allows intermixing of Julia and MATLAB code, e.g.:

julia> mat"""
       x = 1;
       for i = 1:10
           x = x + 1;
       end
       """

julia> @mget x
11.0

It also allows you to use interpolation, avoiding @mput and @mget entirely:

julia> x = 20;

julia> mat"""
       for i = 1:10
          $x = $x + 1;
       end
       """

julia> x
30

or:

julia> v = [1, 2, 3, 4, 5];

julia> mat"$(v[1:3]) = $(v[5])*2"

julia> v
5-element Array{Int64,1}:
 10
 10
 10
  4
  5

Finally, if an expression sets ans, mat returns it, so you can write:

julia> mat"nchoosek(5, 2)"
10.0

There are a couple hacks to get this to work. First, I couldn't figure out how to get string interpolation in a custom literal without calling parse on the corresponding ordinary string literal, so that's what we do. Second, the code to determine whether a variable is assigned, referenced, or both can probably be fooled, although it's hard for me to come up with a case that would actually break at the moment.

If this sounds good to others, we might consider deprecating @matlab in favor of it.

[simonster]

I realized this doesn't work for multiple assignment (e.g. mat"[$a, $b] = [1, 2]"). It may need to be smarter.

edit: That required more code than I would have liked, but it should work now.

[lindahua]

This is a great idea.

However, I would rather keep both and see which one people generally prefer.

[simonster]

Okay, I'll leave in the documentation for the other methods.