Render tabulations

[gilbertfl]

In PR #10, erikn proposed a partial implementation of the horizontal tabulation commands.

This issue is to implement de complete mechanism for horizontal tabs. These seem very useful in the context of cash register receipts, eg. to align line totals together.

To implement this, we must deal with 2 new ESC-POS commands:

• ESC D (set horizontal tab positions)
• HT (horizontal tab)

We must also extend how we deal with ESC @ to reset the positions set with ESC D.

Links to the Epson APG:
ESC D
HT