Handle hyphens in dataset()

[mortenpi]

When calling dataset("dataset-name"), it actually has its own regex it checks when it parses it (because of query parameters), and it doesn't currently handle hyphens.

I now try to reuse the regex string for the dataset name, rather than having two separate regexes that do partially the same thing.

It should also be backported to 0.2.

---

This is how the new regexes print:

julia> DataSets.DATASET_NAME_REGEX
r"^
[[:alpha:]]
(?:
    [-[:alnum:]_]     |
    / (?=[[:alpha:]])
)*

$"x

julia> DataSets.DATASET_SPEC_REGEX
r"^
([[:alpha:]]
(?:
    [-[:alnum:]_]     |
    / (?=[[:alpha:]])
)*
)
(?:\?([^#]*))? # query    - a=b&c=d
(?:\#(.*))?    # fragment - ...
$
"x