Add support for conditions: boolean and comparisons

Add support for boolean conditions and comparisons

The conditions take effect only the XML converter. The pretty-printer
will continue to print a reformatted copy of the input.

Add support for including documents

Add capability to include SAMx documents

SAMx includes are automatically expanded when converting to XML. They
are not expanded when pretty-printing the SAMx document.

Add support for external code and URLs in flows

v0.1.3

Preserve the indentation of external code

Improved packaging of releases; add samples

v0.1.2

Include test samples in package

Initial parsing of a subset of SAM and pretty-printing

v0.1.0

Add introduction and license information