Adds option to print result to stdout

Fixes #18

Prints stacktrace on error to stderr

Also, prints --version output to stdout

Adds -v flag to CLI

Adds support for LHS without predicates and numeric literal on RHS

Fixes #13:

  * Supports specifying dependent variables without predicates
  * Dependent variables without predicates can only be used when
    comparing them to a numeric literal. This is out of simplicity,
    i.e., it doesn't make sense to compare one variable to itself. This
    might change when support for multiple variables is added by #12
  * Dependant variables can only appear in LHS

Fixes #14:

  * Modifies the parser so that numeric values can be given in the
    comparison clause
  * numeric literals can be referenced on RHS only

Fixes main so it prints usage() correctly

Also, log.Fatalln() is used instead of log.Fatal()