Add support for any kind of content in make.conf

This commit allows us to put anything in make.conf, not only an
expression like VAR="value", which is handy for lines like
`source /var/lib/layman/make.conf`

fixes #60
Add support for lists in portage::makeconf content
Updates in README for the portage::makeconf changes
Merge pull request #102 from tampakrap/makeconf_list

Updates in portage::makeconf