Fixed interaction between 'ignorePrefixedFiles' and 'main_file' settings

See issue #60

This fixes an issue where when `ignoredPrefixedFiles` is *true* nothing is compiled, even if `main_file` is also specified. This seemed counter-intuitive and a bit confusing since the immediate assumption is that both must be set to *true* to get the behaviour where `main_file` is compiled but not its imported files (usually prefixed with a '_'), which is actually achieved just by specifying `main_file`.

An alternate fix for this might be to change `ignoredPrefixedFiles` from a *boolean* to a *string* and have the user specify the prefix to ignore, with an empty string meaning not to ignore any files.