Add custom WebSocket parameters

[cquesnel]

Allows users to pass custom WebSocket parameters allowing to pass request headers and cookies.

Fixes #495 and #496

[cquesnel]

@chkr1011 The build won't pass because of an expired certificate in appveyor

[chkr1011]

Thank you for your contribution 👍
Good work! But I have some things to change:

• Please use a default parameter instead of an overload. This is used for all other methods as well. So using a overload will break this pattern.
• I am not sure but is the Cookie container also available for UWP? Did you build for all supported platforms?

Best regards
Christian

[cquesnel]

@chkr1011

• I changed the method to use a default parameter
• For the cookie container, I simply exposed a field that was already present, so I'm pretty sure it's compatible. I did build the whole solution so there shouldn't be any problems there.