config: ignore leading/trailing spaces in ';'-delimited lists

In [1] Ciprian observed that it was easy for users to mistakenly
introduce leading and trailing space to new.tags when editing a
notmuch config file. This commit strips spaces on either side of the
';' delimiter when splitting.

In principle it would be possible to support tags (or other config
values) with leading or trailing spaces by processing '\s' escapes in
the input string. Currently such processing is not done.

[1]: id:CA+Tk8fzjPLaEd3vL1f9ebk_bF_RV8PDTLzDupraTkCLCpJAmCg@mail.gmail.com