Support plain-text and other sessions

[countvajhula]

Currently, the buffer name, file extension and major mode are hardcoded to Racket/.rkt/racket-mode. This should be changed to be based on the chosen template. That is, we could rely on the template extension to determine all of these values.

• Use "Mindstream" as the buffer name instead of "Racket"
• Infer .rkt in the template to mean racket-mode, .el to imply emacs-lisp-mode, etc.
• So either maintain a hash mapping file extensions to major mode, or (better) find a way to query Emacs's existing mapping of file extension to major mode

Also see greghendershott/racket-mode#628