Refactor: Convert some Option[string] to string

[ee7]

There isn't much reason to use Option[string] for exercise and probSpecsDir - we can just test whether the string is empty.

This also allows us to consider setting the default value of probSpecsDir to .problem-specifications in the future, which better models the current behaviour.

Note that (both before and after this commit) if we run any of:

canonical_data_syncer -p=
canonical_data_syncer -p
canonical_data_syncer -e=
canonical_data_syncer -e

Then we see an error message like:

Error: '-p' was given without a value

[ErikSchierboom]

I do feel that we're sacrifing readability a bit here. conf.exercise.isNone to me is more descriptive for what is happening than conf.exercise.len == 0. But I don't feel strongly about it.

[ee7]

I actually prefer it this way.

I can agree that .isNone and .isSome are "more descriptive" than the corresponding .len == 0 and .len > 0. But I personally find the latter more "readable" - it avoids the noise, and otherwise a reader might wonder whether there's some subtle behaviour that makes Option necessary. For example, they might ask: "do we distinguish between some("") and none[string]?"

It's also good enough for Nim and Nimble (see in particular nimbleDir, which is a similar "option with a value").

There's also the argument of "if probSpecsDir is Option[string], then check should be Option[bool]".