Implement loading progress indicator

[jsumners-nr]

When the application starts up, it either parses a log file (which can be quite large) into memory, or it queries an existing cache file for a set of logs. Both operations can take a noticeable amount of time to complete. We should implement a threaded loader so that we can display some sort of progress indicator. This will give the user confidence that something is being done, and some idea of how long it will be before they can start searching logs.

[jsumners-nr]

I am thinking we want to define something like:

type DataReader interface {
	io.Reader

	TotalLines() int
}

And then implement two readers like LogFileReader and DatabaseReader. This could make it easy to use a single routine for initial loading and progress indicator updating.