Config support vi-mode and emacs-mode, F4 switching them.

[laixintao]

• [ ] config option;
• [ ] F4 switch like mycli;
• [ ] display in bottom;
• [ ] update readme, key binding.

[laixintao]

https://github.com/dbcli/mycli/blob/master/mycli/myclirc#L49

[psacawa]

I'd like to take a stab at this. To reconcile the bottom tool-bar space, I have the following plan:

• A new vi_mode config option
• The bottom_toolbar config option controls whether the toolbar is rendered.
• A new help_location option controls how the command help messages are shown. It takes one of the three options: bottom, meta, <empty>.
  • In the case of meta, the help message is rendered in a prompt_toolkit meta bubble, like in an example. In my view, this is how it should be, as the bottom toolbar is not large enough anyway.
  • In the case empty string case, the help is not displayed.
  • In the bottom case, the help message is given precedence over the vi mode toggle display, if a command is already on the line.
• In all other matters, follow the lead of the pgcli.

Let me know if you agree with this.