Allow overriding of SlickGrid options

[snth]

I wanted a way to override the forceFitColumns parameter and this PR works for me.

Example usage:

display(qgrid.SlickGrid(grid_data, options={'forceFitColumns':False}))

[ssanderson]

This shouldn't be a dict: see http://docs.python-guide.org/en/latest/writing/gotchas/#mutable-default-arguments

[snth]

Thanks, while I agree that this is a common gotcha, in this case I had thought about it and concluded it's fine. options is only read later (in self.options.update(options)) and not modified, hence I don't believe your concern is an issue here. I thought that this usage is more self-documenting than the standard approach of options=None and then later:

if options is None:
    options = {}

I'm happy to change it to the latter though if you are concerned but I don't think it's necessary.

[ssanderson]

@snth I realize the particular usage is safe here, but it's easy for someone to make a change in the future that make it unsafe. I switched over the default to None in #24 at any rate.

[TimShawver]

@snth this looks awesome...such a useful change. Once you address Scott's comment I'll be happy to merge it.

[ssanderson]

@TimShawver if you're happy with the functionality here we can merge and iterate on the specific implementation.

[ssanderson]

@TimShawver @snth I made a secondary PR on top of this one at #24. I think it's the right way forward, but I'd like to get your guys' feedback.