change feature arguments to pattern

[kbenoit]

only once #838, #837, and #836 are solved, I propose we change the main matching arguments to pattern, from features or keywords.

So this would be:

• tokens_select(x, pattern)
• tokens_remove(x, pattern)
• tokens_compound(x, pattern)
• dfm_select(x, pattern)
• dfm_remove(x, pattern)
• fcm_select(x, pattern)
• fcm_remove(x, pattern)
• kwic(x, pattern)

Also:

• deprecate the features (and keywords for kwic) arguments in the same way that we did this for the camelcase arguments in tokens(). So they will still work but issue a message.
• create a central man page for pattern, and update the existing one for phrase (and these might be the same page)

Why make this change?

• more consistent across functions
• more descriptive than features, which is really specific to a dfm construct, but we have used it to refer to tokens, sequences, and pattern matches generally
• pattern is consistent with the usage stringr and stringi