Invoke pkg-config at most twice

Changes the RC file format so that
  pkg-config --cflags foo
  pkg-config --libs foo
are merged into the single line
  pkg-config foo
with the previous two syntaxes detected as errors.

Rather than invoking pkg-config as each line is encountered,
the packages are gathered together and pkg-config is run
exactly twice.  Should marginally speed startup when large
configuration files are used as well as merging resulting
flags and permitting pkg-config goodies like conflict
detection.

Reasonable attempt made to ensure error messages are helpful.
In particular, that they enumerate exactly from where each
package was requested.

Closes #9.
Permit -p to request packages from CLI

Makes using HERE documents with things like the GSL
much nicer.

Closes #8