Add select/highlight to shown text

[mktip]

This request might be out of scope for the project overall, and if that's the case please ignore it.

For the same illustrative purposes mentioned in #65, I think it would be nice to have some construct to highlight a specific portion of the text shown at a specific point in the demo. Maybe some regex could be specified and used for this?

Also for highlighting it does not need to be like the usual text highlighting, since its a video maybe the highlighted word could glamorously glow for instance, or blink, or get underlined, or have an aura around it, etc.

It might be something as follows for example to highlight the word "VHS" that was previously typed to the screen:

# Where should we write the GIF?
Output demo.gif

...

# Type a command in the terminal.
Type "echo 'Welcome to VHS!'"

# Pause for dramatic effect...
Sleep 500ms

# Remember the name!
Highlight "VHS"

...