New function google_fonts() to assist installing Google Fonts
to the local machine, suggested by
@thomas-neitmann
(sysfonts issue #12).
Fixed a font loading issue when a font directory exists but with no files inside, thanks to the debugging by @yihui and @XiangyunHuang (showtext issue #30).
showtextdb will now test for write permission before installing and extracting font files.
New function font_install() to install fonts to the local machine.
New function font_installed() to list locally installed fonts.
New functions source_han_sans() and source_han_serif()
to assist installing the Source Han Sans/Serif fonts.
The initial version that separated the font file from the showtext package.