Implement `fmf find`

[psss]

As outlined in #43 it would be good to have an easy way to discover all metadata trees present under given directory on the filesystem. This would allow us to easily process them one-by-one. A new subcommand find should be added which will list root directories of all subtrees found. Additional options can be provided later to optionally list the metadata as well. Example output:

> cd /usr/share/doc/fmf-0.5/
> fmf find .

./examples/child
./examples/scatter
./examples/deep
./examples/touch
./examples/wget
./examples/merge

[jscotka]

@psss just note. I expect that this will be part of Tree API (I can imagine for example Class method) or utils, not just implemented it as part of CMD line tooling.
Or maybe it is good time to talk about these plugins. because #46 could be also solved as an plugin/extension to FMF