[feature] Extended directory support

[Ignatius18652]

Currently bookokrat only supports documents within the same directory, and opening documents from outside this directory involves closing bookokrat, switching to that directory, and opening bookokrat. It would be good if bookokrat would support subdirectories within the directory, and list these subdirectories above the documents in the library window, and above that a <- to return to the upper directory. Similar to NERDTree in vim.

Or even better but more complicated, give bookokrat full access to ~/ and the ability to navigate through directories to find documents to open. Like how NERDTree does it.