Support wrapper field in Installer data parsed from User Agent

[chrahunt]

As mentioned by @ncoghlan in pypa/pip#5424 (comment):

submit a PR to update linehaul's UserAgent schema to include an extra optional wrapper = pyrsistent.field(type=Installer, factory=Installer.create) field

The motivation is to allow wrappers, like pipenv, to measure adoption of their tools.

The project has since switched to attrs, but the idea is the same. The change should be somewhere around here.