RFC: Added basic support for searching and viewing REPL history

[dahtah]

This is an experiment (code is pretty dirty) but I think the functionality could be useful. I added some functions that make use of the command history recorded by the Julia REPL.

• julia-snail-search-history-and-insert is a replacement for C-r and C-s in the REPL. It uses ivy to search through command history: set up using e.g.

  (define-key julia-snail-repl-mode-map (kbd "C-c C-s") #'julia-snail-search-history-and-insert)

  and hit "C-c C-s" in the REPL to search, Enter to insert

• julia-snail-insert-history inserts the last n line of REPL history into the buffer. It happens to me all the time that I type a bunch of stuff into the REPL, then need to transfer it into a script. This function is meant to help with that.

  (define-key julia-snail-mode-map (kbd "C-c h i") #'julia-snail-insert-history)

  Hit C-c h i to insert the last line, prefix with C-u to insert the last n lines (eg C-u 4 C-c h i)

• julia-snail-history-occur simply runs occur on REPL history (define-key julia-snail-mode-map (kbd "C-c h o") #'julia-snail-history-occur)
• julia-snail-view-session-history opens a buffer with the entire REPL hsitory for the current session (define-key julia-snail-mode-map (kbd "C-c h s") #'julia-snail-view-session-history)

[gcv]

I like this! It's a great set of features to add to Snail.

For now, I have two concerns about the implementation.

1. Instead of adding a hard dependency on Ivy / Counsel, we should use a user-configurable read function. That way, people can use Ido, Ivy, Helm, Selectrum, or just vanilla completing-read. See https://github.com/nex3/perspective-el/blob/2ac6aff/perspective.el#L79-L83 for an example.

2. Your code uses calls to JuliaSnail.start to delineate sessions, but that doesn't work with multiple REPLs, does it? Snail supports multiple simultaneous REPLs, and the session history feature won't work right if they're used. Something can probably be done by tweaking the JULIA_HISTORY environment variable on startup, but doing it cleanly will require some careful thought. It'd be nice not to break history for anyone who mixes Snail and non-Snail Julia use.

[gcv]

Replaced with #58.