Allow deeper REPL config + dbcli config covergence

[psacawa]

The dbcli projects all use the the prompt_toolkit module, as do other popular packages like ipython. This module exposes a lot of possiblities to configure prompts, the majority of which are not used by pgcli, or the other dbcli projects.

Ipython enables users to configure the prompt in an arbitrary way, by placing code modifying the prompt application in so-called startup files in their shell profiles. For example, I can replicate the core behaviour of the popular vim plugin vim-surround with a startup file. I can also use fzf for fuzzy history search, and install my own custom key bindings. This results in a great REPL-oriented workflow. By way of comparison, dbcli projects have very limited capabilities to customize the shell.

The request is twofold:

• The dbcli projects should strive to enable that degree of customization, perhaps via scripts in a config folder that run on startup
• There maybe should be some form of configuration convergence among the dbcli projects. It should be possible to use the same configuration scripts for pgcli, litecli, and iredis (what I need).

It's unclear how that should be implemented. I am interested in hearing some use cases to understand how this relates to how people use the dbcli projects presently. Once that becomes clear, I'd like to make a PR.

[psacawa]

I have something in mind like the content of this diff in my fork. This dynamically loads all modules in the ~/.prompt_toolkit directory and send the PromptSession the main routine in them. Hypothetically, something like this could serve as a central location for prompt configuration for all prompt-toolkit applications.

For example, ~/.prompt_toolkit can look as follows

/home/psacawa/.prompt_toolkit/
├── 10-fzf.py
├── 20-surround.py
└── 30-misc-keys.py

while e.g. 30-misc-keys.py can look as follows:

from prompt_toolkit.shortcuts import PromptSession
from prompt_toolkit.filters import ViNavigationMode

def main(pt_session: PromptSession):
    """prompt-toolkit key mappings"""
    bindings = pt_session.key_bindings

    @bindings.add_binding('J', filter=ViNavigationMode())
    def next_history(event):
        event.current_buffer.history_forward(count=1)

    @bindings.add_binding('K', filter=ViNavigationMode())
    def prev_history(event):
        event.current_buffer.history_backward(count=1)

This mirrors a configuration scheme used by Ipython. With time, configuration of this sort can be included in all dbcli projects, leading to use advances across the project. Eager to hear thoughts.

[danihodovic]

fzf for history would be wonderful