Exclude repositories from Featured Projects

[Bastian]

We have repositories which are automatically used and often kept up to date, such as AOEpeople/_preview-opensource.
There is no need to show these repositories, so we want some way to exclude them.

Possible usage: have a list of excluded repos, fetch 6+len(list) and filter out repos and display up to 6.

[resah]

Was implemented in #42 and can be configured in config.toml to exclude further repositories.