Add trailing_newline option to log

[capripot]

Describe the change

Adds trailing_newline option to TTY::Spinner#log.

Why are we doing this?

Following discussion on issue #43

Benefits

Allow seamless use of spinner.log("cool feature") but also more complex feature, like redirecting output of other command spinner.log("output\rcountaining carriage returns\r", trailing_newline: false).

Drawbacks

N/A

Requirements

• Tests written & passing locally?
• Code style checked? (rubocop on current master returns 62 files inspected, 102 offenses detected, 64 offenses autocorrectable)
• Rebased with master branch?
• Documentation updated?
• Changelog updated?

[capripot]

We should think of multi-spinners case as well I think