assign defaults to NULL for some arguments to simplify the logic a little bit
just use output = NULL by default
fixes #54: use base::file() instead of file(), because 'file' is an argument of markdownToHTML(), which is missing by default
add a few examples as test cases for #54 and hadley/staticdocs#66
leave some margin (padding) around body, otherwise the HTML vignettes look terrible in RStudio IDE due to lack of margin
white spaces
a similar problem as #54 ('file' argument missing)
roxygenize
bump version