add "clean" option to yujin_init_build

[Marcus Liebhardt]

For multiple reasons I sometimes end up with wrong parameters for the build config.
So far I have to delete all created files manually and redo the build config.

Hence, it would be nice to have a yujin_init_build --clean option to remove all files, which have been created by yujin_init_build before.

[Daniel Stonier]

v0.2.21