added option 'currentLinkMarkup' to MarkupPagerNav module

Hi Ryan,

if pagination is allowed I wanted to print the current pager link with different markup.

In my case as followed:

#### Standard
```
<a href='{url}'><span>{out}</span></a>
```
#### Current
```
<strong>{out}</strong>
```
To reach this I've added an additional option __'currentLinkMarkup'__.

I think this is a common feature to paging navigations?

P.S.
I've removed all trailing slashes, too. ;-)