Proposed changes to the templates branch

[florisvdh]

With reference to pull request #14 (Templates):

• I propose a merge with the inconsolata branch
• several Linux instructions added in README.md
• the number of output formats for simple and elaborate report Rmd templates is extended
• support for those other output formats is improved, through YAML settings and textual explanation
• a toc_name bug in inbo_slides is solved
• corrections and additions in several Rmd templates, such as:
  • subtitle position in YAML
  • linked citations
  • references chapter for inbo_boek
  • Flanders Art font as default for flanders_slides
  • clarifications on language settings

[florisvdh]

I made a choice among the possible colours of the styleguide, for the boxes. Note that in one case, I've taken a white font colour. @inbo/bmk: to be reviewed!

[florisvdh]

With these commits, my intended additions to the templates end (I think - more or less). They originate from my personal adventures in writing a scientific report; I have tried to accomodate things that scientists will need in the _inboboek template. I tried to support both PDF- and HTML-based output formats, especially because HTML-based formats compile quicker and come in handy for intermediate compilation.

In summary, these are the main extra features compared to the initial state of this pull request - apart from fixing my own bugs:

• support for landscape page orientation;
• add a chapter with explanations on inserting parts, appendices, HTML blocks and other things. Hence this chapter holds a fix for issue #19;
• the originally added pieces on external figures and markdown tables were moved into that chapter;
• change the colours of blocks, and enhance support of markdown features by using the block2 engine.

[ThierryO]

@florisvdh I've reverted that commit.