#1 allow asciiTeX to read from standard input

[retvari]

As the title says: asciiTeX is infinitely more useful (at least for me) if, instead of bailing out with an error, it tries to read from the standard input in the case when no equations/files are given explicitly. For instance, this allows to integrate asciiTeX nicely into my Emacs work-flow: M-x shell-command-on-region "asciiTeX" nicely replaces the equation in the selected region with the ascii art rendering. Could also make asciiTeX a nice org-babel source code style for automatic ascii rendering of math content.

The below patch implements this behaviour. Only code is updated, docs and stuff not.

[retvari]