Support "npm-style" hypen ranges.

[sbs2001]

See https://www.npmjs.com/package/semver#hyphen-ranges-xyz---abc.

[pombredanne]

What about this:

>>> from univers.version_range import NpmVersionRange
>>> npm = NpmVersionRange.from_native("1.2.3 - 2.3.4")
>>> npm.to_string()
'vers:npm/<=2.3.4,>=1.2.3'

[pombredanne]

@sbs2001 closing as this is supported alright