Add/subtract methods for nanoduration/difftime durations

[trevorld]

• As a follow-up to feat: Add 'as.nanoduration.difftime()' #106 I could do a PR to add S4 "+"/"-" methods to add difftime and nanoduration durations together

[eddelbuettel]

See #108, and next time please pool issues that are effectively identical (+/- methods for nano{time,duration} objects) in one if you can. Maybe consider a single PR too for both #107 and #108. Thank you for your consideration, and thanks for offering to do this.