`Options` objects are converted to a string in some circumstances but
this produces output like `#<Options:0x0000000102592c90>`.
For example, formulae.brew.sh contains analytics entries with
incomprehensible names like `adns #<Options:0x0000000102592c90>`.
This shortcoming is apparent when compared to other entries like
`adns --HEAD`.
The `Option` class has a `#to_s` method that returns the `flag`, so
this commit simply adds an `Options#to_s` method that calls `#to_s`
on each `Option` object in `@options` and joins them using spaces.
This should produce more useful output when an `Options` object is
converted to a string (e.g., `--first --second`).
