Mathicsscript

-----

* Allow command-line flag `-c` as an alias for `--execute` (along with `-e` to be compatible with wolframscript
* Better compliance on Windows which are GNU readline-starved
* Better unicode detection
* Accept newer mathics-scanner and Mathics3 versions

-----

* Fix `mathicsscript -f FILE` argument. See PR 26

-----

* All escape sequence for unicode and letter names added for GNU Readline
* Use Mathics Scanner package
* Support XDG-style configuration files

-----

* Fix ``Settings`$ShowFullForm`` now that we use Mathics supports Booleans better
* Show mathicsscript version on startup
* Add ``Settings`MathicsScriptVersion``
* Add all of the WL escape sequences
* Add conversions from WL Unicode to standard Unicode
* Shell escapes `!` and `!!` added

Incompatible changes:

* `-e` `--execute` is better suited for embedded use. It adds `--quiet` and shows just evaluation output as text

-----

* We require Mathics3 1.1.1 for features added in that to support unicode and user-formatting
* Start to support Unicode as a CLI option: `--unicode/--no-unicode`. The setting name is ``Settings`$UseUnicode``.
* Add a lot of Unicode symbols and the WL esc sequences. For example letters with dots under them. These are the "Formal" parmaters/letters.
* Handle Unicode versus WL character code mismatches, , in particular Unicode directed and undirected edges.
* Support for ``PyMathics`Graph`` (to be released on PyPI soon).
* Some XDG compatibility
* Toleratte MS/Windows pyreadline which doesn't handle `remove_history_item`
* Show pygments styles when an invalid one is given
* Use "inkpot" for dark backgrounds and "colorful" for light backgrounds.
* Add ``Settings`PygementsStylesAvailable``
* Add settings.m to holds `mathicsscript`-specific definitions `Settings` and their default values. Settings include
- ``Settings`$ShowFullFormInput``
- ``Settings`$PygmentsStyle``
- ``Settings`$PygmentsShowTokens``
- ``Settings`$UseUnicode`` (also mentioned above)

A lot of code for handling graph formatting is here but will eventually be moved to a backend formattting module which hasn't been written yet.

---------

Split off from plain `mathics` script.

* GNU Readline terminal interaction. This includes
- saving command history between sessions.
- variable completion, even for symbol names like `\\[Sigma]`
- limited ESC keyboard input; for example *esc* ``p`` *esc* is π
* Syntax highlighting using `pygments`.
* Automatic detection of light or dark terminal background color.