fix mxUrlConverter.prototype.convert behavior with vscode (hediet#87)

* fix mxUrlConverter.prototype.convert behavior with vscode

This is done by doing 2 things:

fix isRelativeUrl : vscode use a custom protocol (i.e. vscode-resource://) which is not support by the current mxUrlConverter implementation.

fix baseUrl, relative urls are prefixed by baseUrl which is generated for protocol + host. Both protocol and host value are wrong in vscode context.

* Updates changelog.

Co-authored-by: Henning Dieterichs <henning.dieterichs@live.de>

Merge pull request hediet#85 from hediet/pending-releases/v0.7.0-alpha.6

Release [0.7.0-alpha.6] as [0.7.0]

Prevents draw.io from capturing ctrl-p.

Improves readme. Mentions that .drawio.png is now supported.

Fixes pr link.

Increases version.

Merge pull request hediet#67 from hediet/pending-releases/v0.6.6-alpha.2

Release [0.6.6-alpha.2] as [0.6.6]

Clears etag for normalization.

Adds support for incremental alpha releases.

Updates readme.