Session support

[csabahenk]

This PR introduces sessions, named conversations that can be switched between, continued, reviewed, edited.

A basic session management facility is added, too (listing/renaming/deleting/dumping sessions).

[npiv]

Hi @csabahenk looks great. I'm just on the road so will only be this weekend that I can fully go through and merge it.

The only thing I'd like to add or dig into is if if we can't autoconvert the old format for users that are upgrading. i.e. to avoid having the option --migrate-to-session migrate old saved messages to given session, is thatcould perhaps be automatically detected and executed on first run. Not sure if there is a good hook to detect it though.

[csabahenk]

So, then we can do it like this:

• the sessiion dir can be then just ~/.config/chatblade (what was so far the last conversation fiile), ie. we don't need to add the .d extension for the session dir, as the last conversation file will be eliminated (converted, optionally)
• if ~/.config/chatblade is a file, user will be informed that chatblade will be session enabled now, and they have a choice to
  1. exit, leaving stuff intact
  2. save the last conversation to a session, whereby they need to pick a session name
  3. drop the last conversation
• if choice is ii. or iii., create the session dir and if it's iii,, perform the migration process that's currently implemented for --migrate-to-session

I'll update the PR along these lines, probably also during the weekend.

[npiv]

Was playing with it, and it's a very nice feature. I will be using it myself in future. Will merge it today.

I've made some small tweaks

1. The migration of last is transparent, we just do it. If ~/.config/chatblade is a file we make it ~/.config/chatblade/last.json . I tried various ways to break it, but it always migrates correctly, so I think its most transparent for users, they don't even need to realize.
2. Grouped options a little bit by section, with appropriate header and readme tweaks

To be honest I would kind of like to make this use -s instead of a capital, but not sure what to rename stream to :) will see if inspiration strikes, otherwise leave it as is