Provide downloads

[robo9k]

As mentioned in #28 we should provide downloads for each version.
This requires the following things:

• actually bump version number from time to time
• extend CI to publish artifacts (e.g. on tagged commits, see chip8-vm)

We'd need to make a list of files that would need to be contained in such a download, e.g.

• chip8_ui.exe
• SDL2.dll
• README.md
• LICENSE
• examples/intro.ch8

With the current build matrix we would end up with e.g.

• chip8_ui-0.0.1-i686-pc-windows-gnu.zip
• chip8_ui-0.0.1-x86_64-pc-windows-gnu.zip
• chip8_ui-0.0.1-x86_64-unknown-linux-gnu.tar.gz

This might require editing the README to have separate Downloads, Running chip8_ui and Developing chip8_ui sections.
If the README grows too large we might as well use gh-pages to provide a real project homepage (possibly in addition to any rustdoc of chip8_ui itself).