Build directory creation

[ereOn]

We need the ability to create automatically build directories as part of the variant build process.

The build directory must be parameterizable (i.e one can refer to the context, including matrix variants).

Example:

matrix:
  platform: [win32, win64]
path: build/{{platform}}
script:
  - pwd

Would output:

<project base directory>/build/win32

And:

<project base directory>/build/win64