Make pytest interactive with perseus plugin (PoC)

[b0g3r]

Main idea: as fast as I could create PoC with pytest spanshot tests and interactive diff:

1. Use https://github.com/pytest-dev/pytest/issues/5053#issuecomment-484921801 for turning on input
2. Reuse snapshottest, but implement updating in an interactive way
3. Write tests for it (I have no idea how to do it)

[b0g3r]

A few news:

• [x] I finally figured out how to prompt/read something in/from: https://github.com/pytest-dev/pytest/issues/5053#issuecomment-711165319
• [x] I add snapshottest to a _vendor. Without it snapshottest attached as pytest plugin. Also it will let me gradually replace snapshottest to my implementation in more flexible way
• [ ] I didn't write tests for it and it'll be my next step

Again, PoC should behave like this:

• works as snapshottest, but without update option
• but can be run in interactive mode with diff prompting when snapshot.assert() is failed
• three options in an interactive mode: u[pdate], s[kip], q[uite]