Support of parsing and handling the VTerm (virtual terminal)
control sequences. Actually the code is a conversion of the
libvterm (written in C) to Scheme R6RS.

Current status
==============
* DONE Parsing of control sequences to sexp presentation.
* DONE Conversion of sexp to control sequences
* DONE Parsing of command sequences to sexp presentation.
* DONE Minimal vterm state handling support.
* UTF-8 support:
  * DONE parse/encode utf8
  * TODO screen wide character
* DONE Basic screen handling
* DONE resize
* TODO scroll buffer

You can check the vterm-demo project to check how to use the library
to render the process output on the screen:

    https://gitlab.com/vkochan/vterm-demo
