Support accessors with literal names

Implemented in https://gitlab.com/chris-morgan/buble_/tree/class-accessors-with-literal-names.

This is an issue instead of an MR because GitLab is broken (https://gitlab.com/gitlab-com/support-forum/issues/2547).

e.g. class Foo { get 'foo' () { } }

Computed names are still unimplemented, but literal names now work.