Changes:

• Add layout directive for grid/flow layout of group marks. (Experimental until v3.0.0-beta.30)
• Add title directive for adding titles to group marks. (Experimental until v3.0.0-beta.30)
• Add strokeJoin and strokeMiterLimit encoding properties.
• Add clip as top-level mark definition property.
• Add type checking methods to expression language (isArray, isBoolean, isDate, isNumber, isObject, isRegExp).
• Add type coercion methods to expression language (toBoolean, toNumber, toDate, toString).
• Add copy parameter to JSON data format block. Forces deep copy of JSON, potentially useful when loading memory-resident data.
• Revise Bin transform to clamp values to min/max bins, support anchor parameter.
• Revise Bound transform to support mark clipping.
• Revise Bound transform to propagate line/area/trail mark bounds to all items.
• Fix axis label updates upon data change.
• Fix signal bind DOM element management.
• Update test specs.
• Update JSON schema.