Improvements to welcome screen

[Diggsey]

• Removes some code duplication from term2
• Uses markdown to parse markdown-style text and display formatted output in the terminal
• Implements a word-wrap algorithm
• Shows install options before asking whether to customize
• Uses numbered choices rather than letters

The markdown crate exposes a tokenize method which seems to be exactly fit for purpose. However, it exposes a private type in the return value which makes it unusable. I've opened an issue about it, but for the moment I've had to fork it and use a git dependency.

r? @brson

[Diggsey]

Here's what it looks like:

[brson]

This looks really cool. I love the way this emphasizes key info.

Here are the things I noticed from just running it:

• On Unix, the path to .profile in the pre-install message should be indented two spaces
• The red in WARNING looks dark here. Is there a bright variant?
• The post install "Rust is installed now" message contains a leading "#". Not sure if that's intentional, but it looks out of place because I haven't seen another one.
• The post install message might also bold "run source $HOME/.cargo/env". Maybe also bold PATH the same way as the pre-install message.

I'll give the code a review seperately.

[brson]

It looks like the markdown parsing is not applied to the post-install and pre-uninstall messages.

[brson]

Otherwise this lgtm. Very nice.

[brson]

r=me with the markdown processing applied to all the messages and with the bots passing.

This will break tests in cli-install-interactive for sure.

[Diggsey]

That should be everything (just waiting on tests)