Add :when_started hook

First of all, let's describe `:when_started` and `:before_session` hooks.
The description is provided by John Mair:

* `:when_started` is invoked in Pry.start, and lets you make core
  changes like even change which binding the session starts on;

* `:before_session` is invoked after the binding is already decided upon
  so it's too late to change, but invoked prior to the repl loop
  starting up.

Currently, "pry-debundle" has a problem with `:before_session` hook. The
problem with it is a bit tricky and not really transparent. Let me
explain that by showing a real-world example. I will take
"pry-theme"[1] plugin as an example.

Let's imagine that we have a Rails project and we want to use Pry
instead of IRB. We don't want to include all the Pry plugins that we use
locally into our Gemfile, because they are specific for each taken
separately programmer. Everyone uses his own plugins, so we don't want
to clutter our Gemfile. Now then, we add only "pry-rails" and
"pry-debundle" gems into our Gemfile:

  gem "pry-rails", :group => :development
  gem "pry-debundle", :group => :development

Next, we do `bundle install` and launch our Rails console: `rails
c`. Boom! Did you notice that too? The "pry-theme" doesn't work
properly![2] But why is it so? The filthy worm hides in the boondocks of
the Pry hooks. The problem is that currently "pry-debundle" uses
`:before_session` hook. Every time you load a Rails console, Pry itself
loads only those plugins that are defined in your Gemfile. Fathom this.
*Only the plugins from your Gemfile*, if the file is present. Only
"pry-rails" and "pry-debundle" gems in our particular case. Remember
that we didn't add any of local Pry plugins to the Gemfile apart from
these two? Yes, and "pry-theme" isn't loaded either by now.

What happens next is that "pry-debundle" comes into play. It reloads
your Pry plugins without taking the Gemfile into account. Why? Well,
that's what "pry-debundle" is about. It activates *all* your local Pry
plugins, so there is no need to include them manually to your Gemfile.
Looks about right, isn't it? OK, let's lean back, take a good cup of
strong black tea (with a lemon), get our breath and try summarize
what was said before (in chronological order):

  1) Pry loads all Pry plugins from Gemfile, if any;
  2) "pry-debundle" loads and activates other Pry plugins installed in
     the system.

But it still doesn't work properly. "pry-theme" is messed up for some
reason. What the hell is going on? Good question, sir. Fortunately, I
know the answer. And the answer is that "pry-debundle" lacks the proper
hook to fire its own `debundle!` method. That's very simple:
"pry-debundle" does not take into account hooks like `:when_started`
defined in other Pry plugins. Such hooks are loaded by "pry-debundle",
but never executed.  It's an unreachable code. "pry-theme" uses
`:when_started` hook of Pry to set a default colour theme, but it
doesn't work nicely with "pry-debundle".

What wasn't transparent is the difference between `:when_started` and
`:before_session`. As we can see, there is a big difference between
them.

Fix the problem by adding `:when_started` hook to "pry-debundle". That
would allow to execute `:when_started` hooks of other Pry plugins.

[1]: "pry-theme" is a Pry plugin, which colours Ruby code with help of
     ".prytheme" files.
[2]: There is a relevant issue in "pry-theme" bug tracker. Check it out:
     https://github.com/kyrylo/pry-theme/issues/11

Signed-off-by: Kyrylo Silin <kyrylosilin@gmail.com>
Acked-by: Conrad Irwin <conrad.irwin@gmail.com>