Create installation methods for non-developers

~~Right now you pretty much need to be a developer to start using ActivityWatch.~~ (not anymore!)

One easy improvement we could make is to create a bash one-liner that installs ActivityWatch (much like the one-liner used to install brew). In the future (when we leave beta) we'd likely have to distribute installers/packages.

~~Would require some kind of process management for aw-server and watchers.~~ (we now have aw-qt)

Must do:
- [x] Linux: Build a portable `.zip`
- [x] macOS: Build a portable `.zip`
- [x] Windows: Create a portable `.zip`

May do (secondary goals):

- [ ] Make ActivityWatch start on login for all installation methods (when possible, have a checkbox for those who want to opt out). See #116.
- [x] macOS: Build a macOS `.app` (done, but issues remain)
- [x] macOS: Create a [homebrew formula](https://docs.brew.sh/Formula-Cookbook). Done by @freayd, install using `brew cask install activitywatch`.
- [ ] Linux/Debian: Build a `.deb` ([work in progress](https://github.com/ActivityWatch/deb-activitywatch-bin))
- [ ] Linux: Create an [AppImage](http://appimage.org/)? ([Wikipedia](https://en.wikipedia.org/wiki/AppImage))
- [ ] Linux: Create a flatpak. See #476.
- [x] Arch Linux: Create a PKGBUILD and upload to AUR ([done](https://aur.archlinux.org/packages/activitywatch-bin/))
- [x] Windows: Create an installer using ~~[NSIS](https://sourceforge.net/projects/nsis/) or [WiX](http://wixtoolset.org/)~~ Inno Setup.
- [x] Windows: Create a Chocolatey package (done: https://github.com/chocolatey/chocolatey-package-requests/issues/552)

And finally:

- [x] Put installation instructions in README

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