Add an optional templateLocation parameter

For https://github.com/tpetricek/FsLab/issues/39

What do you think? I add an optional parameter for `Journal.Process` in `Main.fs` and update `processScriptFiles` to support provided template location.

[Here](https://github.com/kimsk/Intro-to-Statistics/blob/master/FsLab%20Journal/FsLab%20Journal/Main.fs#L15) is how I use it with my FsLab Journal. And this is the [generated FsLab Journal](http://kimsk.github.io/Intro-to-Statistics/) (change the header and add a link to Udacity).