Added "fixated commodity pricing"

If you put '=' before an annotated commodity's price, it will cause any
future market valuation of that commodity to use that price, and ignore
whatever changes may have happened since in the market price.  This can
be useful if you are tracking gas expenses based on a standard rate
which, although it changes over time, should not adjust the historical
valuation of how much the gas cost at the time it was purchased:

    2009/01/01 Payee
        Expenses:Gas                100 GAL {=$2}
        Liabilities:MasterCard        $-200

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