0.4.0 release todo

[TomasTomecek]

• [ ] test sen extensively
• [ ] write more tests
• [x] tests
• [ ] introduce as few changes in behavior as possible with comparison to 0.3.0
• [ ] document every behavior change
• [x] autogenerate help for buffers
• [x] provide help for commands https://github.com/TomasTomecek/sen/commit/77e9977ba0a0d8c5d65db844c47f6ba6fd478829
• [x] test and polish new features (sort, browser open)