Bugfix: Sometimes GroupWalker would erroneously report errors if the first pattern did not match but the second did.
Bugfix: salve could previously underreport the attributes possible on an element.
Removed unused variables.
Ignore node_modules.
Divided validate into smaller modules.
Introduced salve-convert.
Travis gives us an idea of what engines salve can work with.
Version bump.
Merge branch 'release/0.15.0'
Merge tag 'v0.15.0' into develop

Version 0.15.0.