Don't source functions multiple times

Fixes #117
Merge pull request #118 from mdlerch/global_fix_multiple_sourcing

Don't source functions multiple times
Checking that b:undo_ftplugin exists before appending to it

This plugin expects b:undo_ftplugin to exists which is not always the case. I made a very simple fix and it's very well possible that it can be written more succintly and idiomatically - but it worked for me :)
Merge pull request #119 from dkvasnicka/patch-1

Checking that b:undo_ftplugin exists before appending to it
Update information about vimrplugin_show_args.
Switch to rmarkdown for rmd files

rmarkdown essentially is a wrapper for knitr and pandoc
Remove slidify support.

I am suggesting getting rid of slidify support because a) creating html
slides is easy enough with rmarkdown, b) development on the slidify
package seems to be slowing down, and c) adding support is easy enough
for the user to define their own RAction() call.

Also, this makes the code base cleaner with just a single vim function
responsible for all rmd markup.
Merge pull request #121 from mdlerch/rmarkdown_support

Switch to rmarkdown for rmd files
Merge remote-tracking branch 'upstream/master'
Find indented code chunks.

In Rmd, code chunks can be preceded by white space.
Merge pull request #123 from mdlerch/indent_rmd

Find indented code chunks.
Merge remote-tracking branch 'upstream/master'
g:vimrplugin_assign = 2 swap precedence of '_' and '<-'
bugfix: insert '_' after a rString or rSpecial
short circuit to behave same way as prev bugfix
cleanup to match master on if statement
Merge pull request #126 from pimentel/changeUnderscoreBehavior

let g:vimrplugin_assign = 2 swaps precedence of '_' and '<-'
vimrplugin_assign now accepts values 0, 1 and 2.
Merge remote-tracking branch 'upstream/master'
Note on problematic mappings.