Ioslides levels

## Change
Added the slide_level  argument to the  ioslides_presentation function

## Rationale
The argument allows to specify what header level in the markdown should be used as a slide title. The behavior mimics the slide_level argument of the beamer_presentation function.

## Implementation
The slide_level argument is passed to the _lua_ file and added to the list of global variables.
All headers below the slide_level are considered as _segue_ title slides. The headers corresponding to the
the slide_level are considered as slide titles and used as slide separators.

In order to distinguish between _segue_ slides coming from different header levels a new class has been added to the slide class list: _level_**X**. Where X stands for the header level. It is therefore possible to create CSS styles specific for each type of _segue_ slide.

The _tests/testthat/test-ioslides.R_ file contains the relevant unit tests.