Add option to convert_file API to return output as a string

[mojavelinux]

The convert_file API assumes that if you read the input from a file, you also want to write the output to a file or IO object. However, this is not always the case. Sometimes, you want to return the document to perform further processing on it. The :to option should accept a value that indicates the output string should returned instead of written to a file or IO object.

The values being considered are as follows:

• false
• :string
• nil (this value may be too ambiguous)

[mojavelinux]

I decided to go with any falsy value for now.