configurable GridItem tick spacing and pen color

Hi,

this patch adds configuration options for GridItem grid and text color, and options for custom grid line spacing.

It also makes GridItem honor the 'foreground' config option.
Dear Luke

I don't know why but I have started getting these emails at intervals of 
every few minutes.  I suspect for some reason my email address has got 
attached to a circulation list that you have.

They are fascinating but.............

I do get your daily digests (which I value) but these are sent to my 
other email address which is cdm.htc@gmail.com.

Best wishes

Craig Mackay.

>
>
> In pyqtgraph/graphicsItems/GridItem.py:
>
>>          self.picture = None
>> -
>> -
>> +
>> +
>> +    def setPen(self, *args, **kwargs):
>> +        """Set the pen used to draw the grid."""
>> +        if args or kwargs:
>> +            self.opts['pen'] = fn.mkPen(*args, **kwargs)
>> +        else:
>> +            self.opts['pen'] = fn.mkPen(getConfigOption('foreground'))
>> +        self.update()
>> +
>> +    def setTextPen(self, *args, **kwargs):
>
> "pen" doesn't really make sense for text (for example, you can't use dashing when drawing text).
> It would make more sense to use setTextColor and fn.mkColor.
>
> —
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