change verbose option to getOption("verbose")

[kbenoit]

We can centralise this and let users change the option globally, by setting this in each function argument to function(..., verbose = getOption("verbose").

This is not consistent in core R packages, e.g. source() does, but load() does not use the getOption("verbose") in the function argument.

[kbenoit]

functions that use verbose options:

• bootstrap_dfm
• summary.character
• summary.corpus
• dfm
• dfm_lookup
• dfm_select
• dfm_trim
• textmodel_wordscores
• tokens
• tokens_lookup
• tokens_select