Allow for some of the options to be specified in the call to `runtests()`

[fjarri]

This functionality intersects with the INI-file (see issue #5), perhaps we won't even need the INI files in the end. The runtests.jl will effectively be a configuration file itself.

[fjarri]

Currently either command-line options or in-program options can be active. A proper implementation, that is, command-line options overriding in-program options, is blocked by ArgParse PR #47

[fjarri]

The fix is somewhat hacky, but it works. ArgParse is not particularly active, so the PR might take a while.