Features/custom cell template

Here's a new feature to make ng-cells able to use custom angular templates in each cell.

Now I know that it's not good to do this for the whole grid, but I still find myself in a scenario where we have to make action buttons on the side of a table.
So in my opinion, this new feature really helps to keep things tidy and extensible.

**Documentation**

*When defined on a range*
`custom-cell-template` Render a custom Angular template in the cell instead of using `custom-html-fn`. <br/>
Accepts types: 
    * `string`: URL of an Angular template. E.g. `custom-cell-template="'template.html'"`
    * `Function`: function that returns the URL of an Angular template. 
    E.g. `custom-cell-template="getTemplate"` <br/>
    Function signature is `function(rawData, row, col, formattedValue, scope)` where
    `rawData` is an object that describes the table cell.
    `rawData.data` points to the actual cell value.
<br>
**Note:** The cell value is stored in `scopeExtension.rawCellData` and can be accessed in the Angular template as follow:
`<span style="color: red">{{scopeExtension.rawCellData.data}}</span>`. <br>
See test case ./test/custom-cell-template.html for a full example.