Add ability to remember command line inputs in preferences
Modify plugin decoders to read header but remember lines

We had this problem with tribble codecs before, because one never knows how long a header is, typically they just keep reading until they hit lines which no longer look like headers. This often means missing the first data line. This was solved by opening a separate stream on the file. We can't do that for plugins which read from stdout, so we create a custom LineReader which stores each line once it's been read for later retrieval. Not sure how useful this is generally but it should work for this purpose.