Implement RPM-free mode

[nforro]

specfile uses RPM for macro expansion and condition evaluation, however, that's not always necessary, hence the force_parse argument to the Specfile constructor that allows to use the created instance even if RPM parsing fails. But since specfile is already capable of parsing macro definitions as well as macro substitutions, we could go one step further and implement a simple, self-contained macro expansion, without using RPM.

[lachmanfrantisek]

Two main benefits:

• No need to rely on RPM.
• Be able to parse parts of specfile or specfiles that are not parsable as a whole (e.g. on a particular system).

This might be highly beneficial for Copr as well.

The solution might be in the form of a different mode people can switch to.

@praiskup will start a community discussion to gather some feedback.