Implement conditional assembly pseudo-operation

[craigthomas]

Many assemblers allow for conditional assembly of statement blocks. This issue will implement conditional assembly by adding a COND pseudo operation that defines the start of a conditional block, and ENDC which defines the end of a conditional block. Conditionals come in the form of:

COND <expression>
<statements>
ENDC

Where:

• <expression> is a traditional expression that contains two values separated by an operation.
• <statements> are the assembly language statements to be included in the conditional block.
• ENDC ends the conditional block.

The conditional block is only assembled if the value of the <expression> results in a non-zero value. Conditional blocks cannot be nested.