Make all functions return characters

They currently returned shiny-classed tags lists
Pass additional argument to control the class of the table
hr is apparently not exported...
Simpler detection of file changes after rendering
More robust error handling using on.exit

Allows to reset the working directory correctly even on error
Improve display_table

Allow HTML code in content and add thead and tbody tags
Allow to choose number of digits displayed for display_table

+ improve documentation, with examples in particular
Make the creation of ul/ol lists a single function
Improve documentation and consistency for rendering

All functions start with `render_`
New function for pure markdown and pass-through html rendering
Improve user messages