rename --function to --only in functions:log (#83)
rename --function to --only in functions:log (#83)

* rename --function to --only in functions:log

* allow user to fetch functions logs from multiple functions

* refactor functions:log command code to be more functional
firebase/firebase-tools,lguzzon/firebase-tools,firebase/firebase-tools,lguzzon/firebase-tools,firebase/firebase-tools,firebase/firebase-tools,firebase/firebase-tools