Add support for Ubuntu 18.10, 19.04 and 19.10

[gleonet]

Swift (only tested 5.2) works perfectly on Ubuntu 18.10, 19.04 and 19.10 but there is no swift releases which contains these Ubuntu versions in their names.

So a small change to add a support for these Ubuntu versions.

[stevapple]

Thanks for your idea! It's great to add support for non-LTS versions. However, since there's no guarantee that those versions work perfectly with all the toolchains for bionic, and it's not quite proper to hard-code specific versions in the program, I may suggest the API in this way:

swiftbox ubuntu 18.04
// or
swiftbox ubuntu 1804

This command may create a .ubuntu-version file with 18.04, which will be read every time to get the version alias.

Also we may use this one to unset:

swiftbox ubuntu current

There's also a more flexible approach by supporting -u flag:

swiftbox get 5.2 -u 18.04
// or
swiftbox get 5.2 --ubuntu-version=18.04

Feel free to share your opinion and thanks for your contribution!

[gleonet]

I like your second approach which is minimal and very versatile.
Thanks for your work

[stevapple]

I’m sincerely sorry for not providing a contribution guide before.

You can now check it out at CONTRIBUTING.md.

[stevapple]

@gleonet Alias support is finally there. Since this is an infrequent usage, there's no built-in command.

I've tested on 19.10 and it's partially compatible with the Swift 5.2.4 toolchain for 20.04 .