Helper methods for common solvers

[fitzgen]

We should add Context::with_z3 etc convenience methods for common solvers that pass the usual flags to get that solver in its REPL mode. This way users don't have to remember and/or cargo cult Context::new invocations.

[pleich]

Since I wrote the methods anyway, I created a small PR to initialize a ContextBuilder with the appropriate defaults in this PR

[elliottt]

Fixed in #52