get knitr doing rmd
basic rmagic argument
add general pre code
code magician to select correct r magic.

Previously, notedown was naively interpreting the language string in
the fenced code block as the string to use for the code block magic.

This was a problem for `R` as the code blocks are often marked

    ```r
    some r code
    ```

which renders as

    %%r
    some r code

whereas the ipython code cell needs the content

    %%R
    some r code

This commit adds an intelligent code magician that chooses the correct
magic for the language alias given. This can be extended to any
language and it will default to just using the alias.

Conflicts:
	notedown/notedown.py
silent knitr execution
doc
add nomagic argument
command line arguments for r-markdown

attempt to fix issue #1

usage:
    notedown example.Rmd --knit --rmagic > example.ipynb
tweak doc
r examples