create `gdt` test runner

[jaypipes]

Create a gdt binary that can be pointed at a file or directory of test scenarios and execute them. This will bring gdt's declarative testing system to communities outside of the Go programming language and allow injection of a functional declarative test runner in agents and other platforms.

The tricky bits are going to be the loading of custom fixtures. Since custom fixtures are written in Go, the gdt runner binary will need to be able to somehow create the gdt context and load the fixture dynamically.

The UX for this gdt CLI binary should look something like this:

# Check that tests are properly formed
$ gdt lint /path/to/test.yaml
# Execute a suite of tests...
$ gdt run /path/to/tests

Obviously, before any of this is possible, we need to create a separate code path for test execution that does not build on top of the Go test tool.