Allow to set empty strings

Something like this in the config file

    test ""

would currently fail with the following error:

   directive "test": want 1 params, got 0

By allowing go-scfg to read empty strings, it would be possible to set
default values, but still allow users to unset those.

Include line number in error messages

Use own word splitting function

scfg word splitting is a bit different than shlex'. Let's just
implement our own parser.

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Initial commit