Support for multiple custom properties

[umstek]

A DXF file can contain multiple custom properties. Currently the parser thinks there's only one custom property and outputs "$CUSTOMPROPERTYTAG": "<Property Name>" and "$CUSTOMPROPERTY": "<Property Value>". The selected custom property happens to be the last one since the others are overridden by the last one. This structure should be something like:

"CUSTOMPROPERTIES": {
  "<Property Name>": "<Property Value>",
  "<Property Name>": "<Property Value>"
}

If this messes up the naming convention, $CUSTOMPROPERTIES can be used.

[bzuillsmith]

Ah, I was unaware of this as there is nothing in the DXF reference. I see this has caught other libraries off guard as well. The format you specify seems like the way to go. I prefer not to have the $ prefix since it is not a direct reflection of the DXF reference. We should probably even just make the key "customProperties" to hold with the convention of other variables in dxf-parser that don't come directly from the DXF reference. This should produce an end result that can be used like the following:

dxf.header.customProperties["MyCustomProperty"]

[bzuillsmith]

@umstek Do you know of any reference for this?
I think what I need is a sample dxf header for testing.