Add option to install only specific CMake targets

[metthal]

RetDec is a large-scale project containing multiple standalone libraries and binaries. It might be useful if we were able to install only certain parts of RetDec, for example just fileinfo and its dependencies.

This might be achieved through COMPONENT attribute of install directives in CMake but I have never tried it myself. (https://cmake.org/cmake/help/v3.13/command/install.html)

[metthal]

I did some investigation and it doesn't look that good because of multiple reasons:

1. It is not transitive. So if I specify COMPONENT fileinfo to install target of retdec-fileinfo and use CMAKE_INSTALL_COMPONENT=fileinfo then only retdec-fileinfo is installed. Nothing else.
2. You cannot specify more than one component in install() directive therefore you can't for example have components for fileinfo, unpacker, ... because they share dependencies.

[PeterMatula]

Solved in #670 using a custom solution - not a very elegant one, but does everything we want it to do and all the ugliness is located in a single file cmake/options.cmake.