tools/pipbot: Refactor help and welcome text

Issue #186.
tools/pipbot: Untabify source
tools/pipbot: Add an interactive REPL

If we invoke pipbot with no arguments, start a REPL session.

Issue #186.
tools/pipbot: Set a global REPL

Issue #186.
tools/pipbot: Don't print logo when in REPL

Issue #186.
tools/pipbot: Print welcome text on REPL

Issue #186.
tools/pipbot: Fix REPL argv assignment

Issue #186.