Add support for machine-specific settings

[smarr]

This PR adds the ability add machine-specific configuration to the config file and then select it by name when running rebench on the command line.

One use could be to run benchmarks with different settings depending on the availability of hardware resources. For example, with a small and a large machine:

machines:
  small-machine:
    cores: [1, 2, 4, 8]
  large-machine:
    cores: [1, 2, 4, 8, 16, 32, 64, 128]

This can then be used by running rebench like this:

rebench rebench.conf -m large-machine

A machine setting is part of the normal composition process of configurations.
It is the new lowest level, i.e., with lowest priority.
This means, that in the above example, a cores: setting in a benchmark would override the setting.

This realizes part of #257.

[coveralls]

coverage: 53.795% (+0.01%) from 53.785%
when pulling c7bb90f on machine-settings
into 1d8b18a on master.