Use fixed window size if it is not available

[philnash]

This package uses the window-size module to make sure the success message formats correctly based on the window size of the user's terminal.

Like in twilio-run/#82, it could break if the window-size module cannot return a width and height.

We should default to a fixed width of, say, 80 in the case where window-size doesn't return a value.

Steps to implement:

• Create a new file in src/create-twilio-function called window-size.js
• implement a get function that returns either the object that the window-size module would return or a default of { width: 80, height: 300 } (including tests)
• replace the use of window-size in success-message.js with this new module

[whitneyrosenberg]

Would it be alright if I took this task?

[philnash]

@whitneyrosenberg go for it!

[philnash]

Closed by #25 .