use current cell's type when inserting

this preserve all previous behavior, but simply adds the ability to
insert cells without specifying a cell type, in which case the cell type
is inferred from the currently selected cell

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

@@ -731,13 +731,16 b' var IPython = (function (IPython) {'
     /**
      * Insert a cell so that after insertion the cell is at given index.
      *
+     * If cell type is not provided, it will default to the type of the
+     * currently active cell.
+     *
      * Similar to insert_above, but index parameter is mandatory
      *
-     * Index will be brought back into the accissible range [0,n]
+     * Index will be brought back into the accessible range [0,n]
      *
      * @method insert_cell_at_index
-     * @param type {string} in ['code','markdown','heading']
-     * @param [index] {int} a valid index where to inser cell
+     * @param [type] {string} in ['code','markdown','heading'], defaults to 'code'
+     * @param [index] {int} a valid index where to insert cell
      *
      * @return cell {cell|null} created cell or null
      **/
@@ -747,6 +750,7 b' var IPython = (function (IPython) {'
         index = Math.min(index,ncells);
         index = Math.max(index,0);
         var cell = null;
+        type = type || this.get_selected_cell().cell_type;
 
         if (ncells === 0 || this.is_valid_cell_index(index) || index === ncells) {
             if (type === 'code') {
@@ -819,7 +823,7 b' var IPython = (function (IPython) {'
      * default index value is the one of currently selected cell
      *
      * @method insert_cell_above
-     * @param type {string} cell type
+     * @param [type] {string} cell type
      * @param [index] {integer}
      *
      * @return handle to created cell or null
@@ -831,12 +835,12 b' var IPython = (function (IPython) {'
 
     /**
      * Insert a cell of given type below given index, or at bottom
-     * of notebook if index greater thatn number of cell
+     * of notebook if index greater than number of cells
      *
      * default index value is the one of currently selected cell
      *
      * @method insert_cell_below
-     * @param type {string} cell type
+     * @param [type] {string} cell type
      * @param [index] {integer}
      *
      * @return handle to created cell or null