Allow CSRF-token to be a function

[eneroth]

• This modifies make-channel-socket-client!.
• When the function is evaluated, it's expected to return a CSRF-token.
• CSRF-token strings are still supported.

This is in reference to https://github.com/ptaoussanis/sente/issues/351.

[eneroth]

Note @ptaoussanis, I can update the relevant sections in the readme as well, I just wanted to get your feedback on this before proceeding.

[ptaoussanis]

@eneroth Hi Henrik, thanks a lot for this! Merging manually now. Apologies for the long delay getting this in!