Add variable for rmarkdown environment

Currently, when one renders an Rmd through vim.interlace.rmd() the resultant objects are not available in the Global Environment.  This is because rmarkdown::render() sets the default environment to envr = parent.frame().  Since render() is called within vim.interlace.rmd(), parent.frame() becomes the vim.interlace.rmd() function environment which is lost after the function ends.

This PR introduces an option to set the desired environment and sets the default to be .GlobalEnv which I believe most users would expect.  It would also be OK with me if this PR is not used and, instead, .GlobalEnv is hard coded in VimCom.

Also, just note that I added the option to the doc in an order that I thought made sense.  When I added the option to the doc, I noticed that the SyncTex options were numbered incorrectly.  They were previously numbered 6.32 and should have been 6.33 so this PR moves them to 6.34.