Load selected feed after a short timeout

Avoid firing multiple server requests if the user is navigating up/down
the tree list with the keyboard. Instead, wait till the user has stopped
navigating for at least 250ms.
Don't preload audio attachments.
Merge pull request #226 from mhaley/patches-no-preload

Don't preload audio attachments.
Merge pull request #223 from mhaley/patches-viewfeed-timeout

Load selected feed after a short timeout