Add option to ignore files while searching

[mgunyho]

Fixes #87. With this, the new default behavior should be to only search for folders.

Todo list:

• [x] Add --option to enable searching for files as well (i.e. the old behavior)
• [x] Update tests
• [x] Change "no matches" message to something like "no folders matching search" if using the new behavior (but not the old one), to avoid confusion
• [x] Update user manual with explanation of files not being matched by default
• [x] Update changelog