Only run CMake if it can be run

[nbolton]

When working on non-CMake projects, the CMake command step is inconvenient.

Make the CMake step optional (perhaps automatically run if there is a CMakeLists.txt file in the root?).

Related:

• Custom commands for other project types #29

[nbolton]

Fixed by #32

[nbolton]

Reopening: what if you have a submodule that has a CMakeLists.txt file?

We should only run CMake if there is a CMakeLists.txt in the root of the workspace.