Pane list

Hi,

I added a syntax file to allow the pane heading to be customisable. It defines a syntax group `slimuxPaneListHeading` that by default is linked to the `markdownH1` syntax group. That will make the current coloring for all users, but now it can be customised using the `slimuxPaneListHeading` syntax group (e.g.: `hi slimuxPaneListHeading ctermfg=red`).

I also made some changes to omit the current pane from the list, as it can mess up the current buffer if you accidentally select the current pane.

The other change was a small annoyance. In the default pane format, there is always a trailing space in the non-active panes, and that triggers the trailing spaces warning that some people have configured. This is illustrated below:

**Before:**
![slimux-trailing-space-1](https://cloud.githubusercontent.com/assets/761254/5428436/7c587bdc-83aa-11e4-81f4-5320ca5e954e.png)

**After:**
![slimux-trailing-space-2](https://cloud.githubusercontent.com/assets/761254/5428438/87ab0f86-83aa-11e4-9ac6-f0959e8a1bf4.png)