Add preliminary support for `doc` USE flag.

Currently only HTML docs are installed and WOFF files are omitted
because of ecompress.

Note: when we want to support PDF and EPUB docs, app-text/pandoc and
dev-texlive/(latex|xetex) are needed.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
Merge pull request #21 from omasanori/iuse-doc

Add preliminary support for `doc` USE flag.