Add `indium-interaction-eval-node-hook` to allow for eval hooks, e.g for overlays

Fix #88

* `indium-inspect-expression`, bound to `C-c M-:`, let the user enter an
  expression to be evaluated and inspected.

Fix #85

* When saving a buffer, do the following:
  - update the script source
  - compile the script
  - update all breakpoints in the buffer

* `set-script-source' is now always performed when a buffer is saved,
  `indium-update-script-on-save' does not exist anymore.

* Update the documentation.

Fix #84

When the page is reloaded, all scripts are being parsed again, with new ids.
Make sure not too keep the old scripts around.

Remove hooks added in tests

The latest version of buttercup requires double quoting of symbols used as
return value in spies.

* Do not attempt to add a breakpoint on a line with an existing breakpoint.
* Do not attempt to remove non-existing breakpoints.

Make sure to delete the entire debugger overlay when the line has been split.
/cc @DamienCassou

When always kept enabled, parsing network requests sent from Chrome slow down
Indium quite a bit.

Most of the time sourcemaps are simply not used, there's no reason to display a
message.

Fix #79

When starting a node process & connecting to it with `indium-run-node`, attach
the nodejs process to the connection.  If the user later starts a new connection
with `indium-run-node`, the first process will then be killed first.

The webkit backend actually implemented the Chrome protocol, which has diverged
from Webkit when Google forked it with Blink.

A new webkit backend is needed to support WebKit browsers (Epiphany, Safari,
etc.).

Fix a typo in debugger.rst

Fix #2

* Make breakpoints a CL struct.
* Streamline the communication between the backend and indium-breakpoint.el.
* Add unit tests.

Attempting to fix #71.

* Refactor all map-* calls to get/set properties of a connection.
* Extract functions to access the current connection.