Discogs-alert

- support for Windows, finally ...
- running `discogs_alert` using docker is now fully supported again
- currency handling is now less aggressive; any errors while converting currencies are simply logged and ignored (as this is one of the most reported issues, and annoying to not see an alert just because the currency wasn't converted)

- a whole sequence of Chromedriver fixes, https://github.com/michaelhball/discogs_alert/pull/66, https://github.com/michaelhball/discogs_alert/pull/68, https://github.com/michaelhball/discogs_alert/pull/64
- properly clean up dangling chromedrivers: https://github.com/michaelhball/discogs_alert/pull/72
- fixes the broken currency API: https://github.com/michaelhball/discogs_alert/pull/71

- & refactors the "alert" module (previously called "notify") to enable easily extending this to more alerting services in the future
- adds an `EnumChoice` click field s.t. these fields can be properly set from the CLI (e.g. min media & sleeve conditions)

& adds the total price to the logs (not yet to the actual alerts sent, that's coming soon!)

untagged-bfc417f30f5ee64629de
- big refactor to make the codebase much more usable & set up for future expansion (e.g. more notification options)
- got Docker working again (thanks ddelange)
- improved the Pushbullet API to use fewer requests

An attempt to fix issues with the closing of Chromedriver, especially on Windows.