Add ability to set the fallback session

[vspinu]

Some ideas:

• Add an interactive command to add a fallback session for context where there is no session available
• Add a defcustom for a default session behavior:
  • ask
  • most recent
  • most recent within project
  • most recent within directory

@bbatsov

[bbatsov]

One other idea (and none of them are exclusive of each other) is to also have "static" session dispatch option, where the users have to select their Sesman session explicitly and it's just used unconditionally until a different session is selected by the users. (similar to the static dispatch CIDER used to have early on)

Having a default session option would reduce the need for this, but having a simple ultra-predictable behavior is not a bad thing in my book.