Enable escaping for hyphens

When an attribute contains an hyphen (" - "), the string needs
to be escaped. If not, graphviz will throw a syntax error.
This diff enables the existing escaping methods whenever an hyphen
is present in the attribute.
It also adds a test to make sure this functionality is present.
Merge pull request #2 from AurelC2G/escape_hyphens

Enable escaping for hyphens