Nested commenting feature

I heard you like pull requests, so here is one with a new feature!

When working with XML, comments can be a real bother. You can't just slap `<!--` and `-->` around an arbitrary selection because you might already have put comments somewhere else.

This plugin ports a nice feature of the Notepad++ "XML Tools" plugin: commenting out a selection will replace any existing nested comments with a placeholder, and uncommenting will restore them so that it becomes a lossless operation.

The feature is used by entering visual mode and then pressing `\c` or `\u` (or something like that if you override the `localleader` setting).

Here's a few examples I've added to the documentation:

Select the settings element by pressing `vat`:
````
<config>
    <!-- Global settings. -->
    <settings>
        <!-- ignores case -->
        <ic>true</ic>
        <!-- shiftwidth -->
        <sw>4</sw>
    </settings>
</config>
````
Press `\c`:
````
<config>
    <!-- Global settings. -->
    <!--<settings>
        <!{1}** ignores case **{1}>
        <ic>true</ic>
        <!{1}** shiftwidth **{1}>
        <sw>4</sw>
    </settings>-->
</config>
````
Now press `V` to switch to visual line mode, select the entire element using `jokk`, then press `\c` again:
````
<!--
<config>
    <!{1}** Global settings. **{1}>
    <!{1}**<settings>
        <!{2}** ignores case **{2}>
        <ic>true</ic>
        <!{2}** shiftwidth **{2}>
        <sw>4</sw>
    </settings>**{1}>
</config>
-->
````
Bam, all nicely commented. Since the code keeps the selection, you can immediately uncomment again by pressing `\u` to see what happens.