This new setting works the same way as the `--all-features` CLI flag
when set to `true`. This makes it possible to set it to `true` permanently,
without having to remember to use the CLI flag when running rust2rpm.
Using .rust2rpm.conf is confusing to new users, because they never see
these files in existing packages and are only seeing magic happen.
This commit changes the default location of the configuration file to
./rust2rpm.conf, and will rename existing .rust2rpm.conf or _rust2rpm.conf
files to the new name if they already exist.
I have also added a little more documentation about the config file itself
to the README.
