Idea: create_episode() should give user option to place episode order

[zkamvar]

At the moment, create_episode() will create a blank episode, and optionally give the user an argument add = TRUE to add the episode to the end of the schedule. There are a couple of problems with this:

1. Users unfamiliar with R may not know about the add feature
2. If users use the add feature without first setting the schedule, then the schedule will only have the new episode added instead of the alphabetical order of the current episodes

There may be situations in which someone would want to programmatically add episodes, but we should probably start with assuming that they will be added interactively.

Here are the possible states:

add = TRUE:  # interactive
add = n:     # place it as the nth element in the schedule (Inf is last and 0 is leave out)
add = FALSE: # leave out

If it's interactive, a user dialog will pop up, displaying the current schedule (even if it is unset) and asking the user to pick a number. The new episode will take that number slot and all episodes below that will be shifted one number. If the user picks 0, then it is not included in the schedule and will be in draft mode.