Fix: add Linux and custom font directories

[vpcano]

Since PR #469 has been opened for 5 years now and is around 100 commits behind, I've opened this PR including also the suggestion from @szduda to use an environment variable to provide custom font directories in addition to looking at each OS default directories.

I've set the following default directories to look for fonts on Linux:

• /usr/share/fonts
• /usr/local/share/fonts
• ~/.local/share/fonts
• ~/.fonts

All unit tests, acceptance tests (behave) and nosetests (coverage) run OK. Docs have also been updated to warn Linux users that PowerPoint fonts might not be available on their distros, and giving some tips on how to get them.

I believe this might solve the following opened issues: #973 and #796 .