^Q would quit even from filter mode

1. Open directories in filter mode
2. Fix arrow keys not working after filter selection (due to invalid cur entry)
3. Support `Insert` key to clear filter prompt
4. Update documentation

Add Debian and Fedora package installation option.

Reasons:

gnome-search-tool is much more powerful than other tools in its category. It
supports numerous search parameters like size, modification date and supports
text search.

1. Strip end spaces and slashes
2. Do NOT change dir if in the same dir
3. Don't go back beyond startup dir with '-'
4. Reset oldpath on cd (other than cd . and ..)
5. Reset filter
6. Update features in README