Add config option for placeholder values

[domsson]

On startup, blocks are often not immediately displayed, because they are still going through their first invocation. Only once they have finished running, can we obtain their output and update the bar with it. This leads to a flash of blank blocks on startup. Additionally, it is possible that a block runs forever (hangs) and therefore never produces output, making for a permanently missing value in the bar.

Both of these issues could be prevented if there was a placeholder config option. The value given here would be immediately displayed where the block output would usually appear until the block has actually produced output itself.

[domsson]

Actually, I'm not so sure this is really needed. I think the problem I saw was from timed blocks (those with a reload value other than 0) that would not be run immediately, but only after their reload time had passed. I believe that was an issue we fixed somewhere along the way. Let's verify all this after the refactoring is done.