Added a PDB implementation
Added some more commands and keymaps
Changed the eval and write keymaps to be more consistent.

Now:
 - `e` and `w` eval and write the word(`e`) or line(`w`) under the
   cursor.
 - Unless some text is selected - in that case they use the selected
   text.
 - `E` and `W` prompt the user to type what to eval or write.