New feature: selectable stylesheets

I just checked into GitHub the code for selectable stylesheets. Until now, a Novelang project could define its own stylesheets, using custom stylesheets. While Novelang can render PDF and HTML with its own, built-in stylesheets, every user probably needs to define his-her own ones. When rendering a document, Novelang attempts to find appropriate stylesheet:

1. In the directory given by novelang.stylesheet.dir system property, if defined.
2. In a style directory under the directory from which Novelang was launched (corresponding to user.dir).
3. Inside Novelang-x.x.x.jar under the /style directory.

"Appropriate stylesheet" means a stylesheet corresponding to the MIME type of requested document: pdf.xsl for a PDF document, html.xsl for a HTML document. That was not flexible enough because the same document of the same MIME type may deserve multiple renderings, like "miser printing", "visually impaired" and "tree-killer". That's where selectable stylesheets come to the rescue. With selectable stylesheets, you give the name of the stylesheet to use. This can be done at query level, or at book level. Let's say this is your project layout, with two stylesheets under the style directory:

/
  book.nlb
  chapter-1.nlp
  chapter-2.nlp
  style/
    html-quick.xsl
    html-beautiful.xsl
    pdf-beautiful.xsl

After launching Novelang HTTP daemon, you can use the stylesheet query parameter to override any other stylesheet name:

http://localhost:8080/chapter-1.html?stylesheet=html-beautiful.xsl

Please note the html-beautiful.xsl path is still relative to the directory containing custom stylesheets! Another place to set stylesheet names is the Book file. Since a Book doesn't know how it will be rendered, you can define a stylesheet for multiple document MIME types. The book.nlb would look like this:

mapstylesheets 
    $html=html-beautiful.xsl
    $pdf=pdf-beautiful.xsl

insert file:chapter-1.nlp

insert file:chapter-2.nlp

I've not tested subdirectories yet but they are supposed to work. Keep in mind: they will be relative to the directory containing your stylesheets. Supporting multiple stylesheets is a necessary step before provinding nice built-in stylesheets to be tried with documents of your own.