Merge pull request #1621 from ivanov/clear-input-prompt-on-clear-all-output

clear In[] prompt numbers on "Clear All Output"

For more version-control-friendly `.ipynb` files, this strips the `In[]` prompt numbers when doing a "Clear all output".  This reduces the amount of noise in commit-to-commit diffs that would otherwise show the (highly variable) prompt number changes.

IPython/frontend/html/notebook/static/js/notebook.js

@@ -874,6 +874,9 var IPython = (function (IPython) {
         for (var i=0; i<ncells; i++) {
             if (cells[i] instanceof IPython.CodeCell) {
                 cells[i].clear_output(true,true,true);
+                // Make all In[] prompts blank, as well
+                // TODO: make this configurable (via checkbox?)
+                cells[i].set_input_prompt();
             }
         };
         this.dirty = true;