Improved ftplugin settings for comments

This pull request addresses two issues with the `comments` and `commentstring` settings.

*   The `commentstring` should have a space separating comment character and comment text. In my experience, style guides universally recommend having a space there (see for example Google R style guide):

        #bad style
        # good style

    The `commentstring` setting is used for *producing* comments, thus it currently produces comments in `#bad style`.

*   The `comments` settings should not have the `b` flag, since the whitespace following `#` is not required syntactically in R.

    Consequences of the current setting are, for example, that it is not possible to reformat

        #paragraphs
        #in this
        #style

    with `gq` because they are not recognised by Vim as comments due to the `b` flag.

For reference I'd like to point to the tpope-maintained Ruby ftplugin which has

    setlocal comments=:#
    setlocal commentstring=#\ %s