Add .tell and .writer syntax for creating Writers. Fixes #920.

[davegurnell]

This adds the following syntax for the Writer monad, as outlined in #920:

log.tell

result.writer(log)

This is my first PR to Cats so there's probably stuff that's wrong in here. I'm happy to fix stuff if required, or close the PR if people think it's not worthwhile. I figured it was easier to have a discussion over a concrete PR than an issue.

[codecov-io]

Current coverage is 88.94%

Merging #922 into master will increase coverage by +0.16% as of 32e2bff

@@            master    #922   diff @@
======================================
  Files          178     179     +1
  Stmts         2122    2125     +3
  Branches        42      42       
  Methods          0       0       
======================================
+ Hit           1884    1890     +6
  Partial          0       0       
+ Missed         238     235     -3

Review entire Coverage Diff as of 32e2bff

Powered by Codecov. Updated on successful CI builds.

[non]

The syntax seems reasonable to me, as do the names. 👍

[stew]

👍 Thanks Dave!