Pweave

* Fix bug in initial whitespace in chunk output
* Remove build artefacts from release tarball

* Fix ptangle script
* Create also intermediate directories for figure directory

* Use IPython kernel to run Python code:
- support for rich output
- support IPython magics
- Improved error handling
* Render tracebacks correctly
* Drop Python 2 support
* Run code using any Jupyter kernel with `--kernel` option
* Output directly to Jupyter notebooks with chunk options preserved
as metadata -> ability to use custom nbconvert templates in addition to builtin formatters.
* Drop uppercase versions on `pweave` and `ptangle` scripts
* Weave documents from URLs
* md2html is the new default format for `.pmd` and `.py` input
* Drop support for specification of figure format. Use default figure format as specified by
output format.

* New pweave option: `output` allows to set the output file
* New better themes for pypublish and an option to choose theme
* New output format: softcover (https://www.softcover.io/)

* New input format: markdown
* New supported script format: Spyder cell markup
* Support new link attributes for pandoc >= 1.16
* pypublish now embeds figures in html output
* pypublish no longer defaults to wrap = False in html output
* Improved test coverage
* More robust script reader, not sensitive to empty lines anymore
* Removed obsolete Julia support.

* Add --latex_engine option to pypublish. Enables the use xetex or luatex
* Bug fixes
- Fix formatting bugs for eval=FALSE 18
- Fix white space error in code chunks for wrap = FALSE 24
- Fix unicode bug with pypublish 21
- Update pypublish template to include textcomp due to changes in Pandoc 23