Add support for multi-source input (Issue #33)

[dobon]

• Add support for multi-source input via using an array of specially formulated objects as input.
• Add spec to test multi-source input covering use of 'page', 'toc' and 'cover' page types, global options, optional arguments and page-specific options. I think this is fairly complete, but more tests could be added.
• Update README.md to explain how to use multi-source input.
• Refactor the options-to-arguments part of code into a helper function for reuse in both single and multi-source input use cases.
• Refactor code location of array of valid ToC options and added array of valid global options. This could be extended to include header and footer options, and page-specific options, and could then be used to validate options.

[dobon]

@devongovett Can someone help me debug why this PR failed the CI test? npm test passes fine on my development environment using wkhtmltopdf 0.12.2.1 (with patched qt) on OSX 10.13.6. I am happy to provide any details that will help.

[zxlin]

@dobon travis was fixed, can you rebase this with master and let it run thru travis again?