Made it possible to pass multiple files to pandoc - fixes #248

[JessicaTegner]

This pr makes it possible to pass multiple files to pandoc in various ways.

Examples (from the updated readme)

import pypandoc

# convert all markdown files in a chapters/ subdirectory.
pypandoc.convert_file('chapters/*.md', 'docx', outputfile="somefile.docx")

# convert all markdown files in the book1 and book2 directories.
pypandoc.convert_file(['book1/*.md', 'book2/*.md'], 'docx', outputfile="somefile.docx")

# convert the front from another drive, and all markdown files in the chapter directory.
pypandoc.convert_file(['D:/book_front.md', 'book2/*.md'], 'docx', outputfile="somefile.docx")

This fixes #248

simonv3

[simonv3]

This looks like it should do what I want it to, thanks @NicklasTegner