Add ability to call external scripts at certain places

[GPMueller]

1. Before any target is parsed
2. Before the build, i.e. before any target is compiled
3. Before any target is linked
4. After the build has finished

Specifying any Python script to be executed would suffice, as any configuration steps etc. (e.g. to create a header with version information) could be done well in Python.

[NOhs]

This is indeed a good idea. I am also considering adding functions in the classes in my branch like:

• run_before
• run_after_compile
• run_after_linking

which are just empty functions. But then we could allow ppl to write custom targets that are derived from the default ones filling in those functions.

[GPMueller]

Added external scripting via targetname.scripts with fields before_compile, before_link and after_build in 79cc6bd.

[GPMueller]

Added on master with 3d12899, including a test.

[GPMueller]

Test passing since 47d3baa (OSX failed due to #30).