Add sentence about U+0000 to spec

Fixes #212.
Off-by-one error in utf8proc_detab
Validate UTF-8 input

Invalid UTF-8 byte sequences are replaced with the Unicode replacement
character U+FFFD. Fixes #213.
Cast void* for MSVC compatibility
Set test environment on Windows

Add directory containing cmark.dll to PATH on Windows.
Run nmake test with /nologo
Fix MSVC options

Also disable some warnings.
Build instructions for MSVC/NMAKE
Add option to select cmake generator

This allows to build under MSYS. The MSYS generator can be selected with

    make GENERATOR="MSYS Makefiles"

But the default "UNIX Makefiles" generator also seems to work.
Add test for input containing null character
Fix .editorconfig

It seems that the vim plugin doesn't support globs in braces.
Merge pull request #217 from nwellnhof/utf8_validation

UTF-8 validation
Merge pull request #219 from nwellnhof/editorconfig

Fix .editorconfig
Merge pull request #218 from nwellnhof/windows_fixes

Windows fixes
commonmark.rb - removed outf.
Run leakcheck in travis build.
Removed leakcheck.md; regenerate alltests.md with Makefile target.
Don't open shared library if --dump-tests or --debug-normalization.
Moved commonmark.rb to its own repo (jgm/commonmarker).
Rename NODE_STRING -> NODE_TEXT.

In JS, use 'Text' instead of 'Str'.

In spec, use "plain textual content" instead of "strings."