allow to use per-project settings file

[blockspacer]

Motivation: file can be used to specify per-project settings. Especially per-project arguments for conan install.

Note: That per-project arguments for conan install must mix with global ones. Because argument order matters - add special var. to global ones {{PROJECT_CONAN_INSTALL_ARGS}} that will be replaced with per-project arguments for conan install

Ideally per-project settings file must be python file, so it will be able to use env. vars etc.

See as example qconan.ini https://github.com/kseinitzer/qconan

[blockspacer]

Related to
#36