Implement `cppargo init` subcommand

[BassedWarrior]

cargo has a init subcommand which allows it to be able to convert an
already existing directory into a cargo crate. Which in this case would entail
converting an already existing directory into a cppargo project.

The cargo init also takes into account the fact that if there are any existing
conventionally named files in the directory, then it keeps them, and instead
includes them in the manifesto as specific paths for the compiler to look for.

Note

As discussed in issue #24, the implementation of alternative binaries being
built from a single project has been deemed outside the scope of the next
version. Hence this issue will instead only focus on dealing with
pre-existing directories, not with declaring an existing main.cpp present
as a binary target with the [[bins]] strategy used by cargo.

This command is more meant to serve the purposes of dealing with pre-existing
directories and filestructures, while not violating any filestructure
conventions. Such as the introduced mandatory src/main.cpp. Which will be
created by default if one is not found to be existing already.

This command could also prove to be very useful if anyone where to migrate
from older versions of cppargo, which most notably didn't have a manifesto.
Reason for which the manifesto will get a bunch of attention in this issue.