Add buffer functions for getting and putting back single characters

* exxe/buffer.h: Add get_buffer() to get the net character from a buffer.
* exxe/buffer.c: Add unget_buffer() to put back a character similar to
ungetc().
Implement environment variable substitution
Implement double-quoted strings

* exxe/parse.c: Parse double quoted strings.
* tests/quoting: Add quoting test cases.