Make window a two-element vector

[kbenoit]

As implemented in tokens_select(), we could add this to the window argument in:

• kwic()
• fcm() - since ordered = TRUE is already like window = c(0, 5). See workaround here.
• tokens_compound() - in fact, this looks like it's already available in qatd_cpp_tokens_compound()

[koheiw]

I don't think we need window to be two-elements in kwic(), but you can do it easily.

[kbenoit]

I agree we don't need it in kwic. Let's close this.