This change adds a `source-filter` option which allows a regex to be
provided to filter the names of keys in data sources. When provided,
only keys matching the pattern will be counted as part of the reported
size. The option can be set via config files or the command line.

Also made --help print to stdout instead of stderr and
don't return error status if help was requested.

Fixes #164.

Currently, only sections and symbols are supported.

Helpful when symbols can contain commas.

I was confused by `-d symbols` was not working as advertised.

Also added some helpful CMake defaults.
- build type defaults to RelWithDebInfo.
- downloads Git submodules automatically.

Also added license headers.