Do not display stdout/stderr from browsers

Redirect stdout to /dev/null so messages like 'Created new window in existing
browser session.' do not appear when openning a new tab.

Redirect stderr to /dev/null so errors when starting some browsers do not
appear.

Signed-off-by: Antoine Tenart <atenart@n0.pe>
make the web_search function more generic

Signed-off-by: Antoine Tenart <atenart@n0.pe>
add duckduckgo as a default search engine

Signed-off-by: Antoine Tenart <atenart@n0.pe>
call the open command at only one place

Signed-off-by: Antoine Tenart <atenart@n0.pe>
[lib][completion] enable user completion by default