Add charset editing feature.

Add charset editing feature.

The syntax to add any characters to the charset table looks like:

<match target="scan">
    <test name="family">
        <string>Buggy Sans</string>
    </test>
    <edit name="charset" mode="assign">
        <plus>
            <name>charset</name>
            <charset>
                <int>0x3220</int>    <!-- PARENTHESIZED IDEOGRAPH ONE -->
            </charset>
        </plus>
    </edit>
</match>

To remove any characters from the charset table:

<match target="scan">
    <test name="family">
        <string>Buggy Sans</string>
    </test>
    <edit name="charset" mode="assign">
        <minus>
            <name>charset</name>
            <charset>
                <int>0x06CC</int>    <!-- ARABIC LETTER FARSI YEH -->
                <int>0x06D2</int>    <!-- ARABIC LETTER YEH BARREE -->
                <int>0x06D3</int>    <!-- ARABIC LETTER YEH BARREE WITH HAMZA ABOVE -->
            </charset>
        </minus>
    </edit>
</match>

You could also use the range element for convenience:

...
            <charset>
                <int>0x06CC</int>    <!-- ARABIC LETTER FARSI YEH -->
                <range>
                    <int>0x06D2</int>    <!-- ARABIC LETTER YEH BARREE -->
                    <int>0x06D3</int>    <!-- ARABIC LETTER YEH BARREE WITH HAMZA ABOVE -->
                </range>
            </charset>
...