[CLI & VS CODE UPDATES] New features for course materials

[oscarlevin]

The PreTeXT CLI is now up to version 2.10.1; you can get the most recent version by running `pip install pretext --upgrade`.

The most recent version of the VS Code plugin "pretext-tools" is no 0.23.0 (just updated today).

I wanted to draw attention to a few new features that are available with these releases.

1. You can now run `pretext new course` to set up a pretext project with components that I think might be useful if you are using a pretext "book" as a set of course materials.  There are a few templates for activities (worksheets) and notes.  
2. Say you write a worksheet for class and want to build just that worksheet.  The course template is set up so you can do this (and still import the contents of that worksheet in your larger document).  A new (experimental) feature of the CLI is that you can build a single pretext file without needing a target in project.ptx for it, by running something like `pretext build ./source/activities/magic-beans.ptx`.  Doing so will produce a pdf of just the worksheet titled "magic-beans.ptx", put directly next to the ptx source file (the way latex compiles a file).
3. If you don't want to remember that command, you can use VS Code to build the "current" file this way: just open the command palette (ctrl+shift+p) and look for "PreTeXt: Build active file as standalone document".
4. Do you have a LaTeX worksheet you want to convert to PreTeXt?  Or perhaps a worksheet written in markdown?  There are now much improved "inline" converters for going from LaTeX or Markdown to PreTeXt.  Select the lines you want to convert, and look for the appropriate command in the command palette (search for PreTeXt:  Convert...).  

All of these features are experimental (the interfaces might change) and can definitely be improved.  If you have troubles or ideas for improvement, please post on pretext-support or make an issue on the github repositories for the CLI and pretext-tools:  https://github.com/PreTeXtBook/pretext-cli/issues and https://github.com/oscarlevin/pretext-tools/issues.  Looking forward to hearing your feedback.