Previously we used midnight, but this meant that the entire day
specified by `end_date` would be excluded, which is counterintuitive.
Instead use a second before midnight to include the rest of that day.

Instead of doing `use meetup::client::Client`, do `use meetup::Client`.
Makes it less redundant.

Make these values user-modifiable finally.

Since we've set up required arguments, 'getopts' fails fast and
complains about missing required args even if the only one used was `-h`
or `--help`. Why should you care about required arguments? You wanted
help in the first place, how are you going to know?

Thankfully, this message from 'prasannavl' pointed me in the right
direction:
https://github.com/rust-lang-nursery/getopts/issues/46#issuecomment-343842947

If there's an option parse failure, we can just look for the help flags
in the args vec to see if we should display the help.

These will be passed to `build_rss::write_feed`.

Well, turns out that the RSS item `description` field uses HTML, so my
`\n`s didn't actually change the formatting in a real RSS reader. Use
HTML tag formatting instead to get our description header with date and
venue to look right.

If either the description or both the date & time and venue were empty,
a couple of newlines would unnecessarily be added to the `Item`
description. This gets rid of them.

Move the join logic from `description_header` into a new function so we
can reuse it.

Instead of the verbose and unclear appending, use a `join` to add the
newline. Filter out empty strings to ensure the newline doesn't get
added if any of `where` or `place` are empty.

Needed to convert the slice strings to owned strings to deal with this
error:

    error[E0599]: no method named `join` found for type `std::vec::Vec<&std::string::String>` in the current scope
      --> src/meetup_rss.rs:92:10
       |
    92 |         .join("\n")
       |          ^^^^