First copy reference files then build home

[arkadiuszbeer]

If you have an image in README.md, then build_site will return a warning message on build_home function.

Warning message:
Missing images in 'README.md': 'reference/figures/image_name.png'
ℹ pkgdown can only use images in 'man/figures' and 'vignettes' 

build_reference function should be run before build_home function call.

[hadley]

I think a slightly better fix might be just to call copy_figures() in build_home_index(). It will potentially copy things twice but it's so fast that it won't matter.