Support user-defined default context/device

Sometimes it is desirable to have CL-accelerated libraries initialize with user context managed by some outside application. But Boost.Compute currently hides global state of `context/device/queue` from users; there is no way to let user override e.g., their own global default context.

Because Boost.Compute already maintains global state of `context/device/queue` inside static functions, I think the best case scenario is being able to assign global CL context (derived from preferred CL compute device) without relying on parsing through pre-defined environment variables which `default_context()` implements.

One of the main benefits is to make direct access of device memory between shared objects/DLLs easily achievable. When some main application initialize two libraries `Foo` and `Bar`, those two libraries also initialize all internal buffers with global user-defined context. That way, data produced by library `Foo` can be directly consumed by library `Bar`. If not being able to set global state it would take much effort for library authors to add additional input argument for virtually every Boost.Compute call.