Introduce clojure-ts-completion-at-point-function

[rrudakov]

Close #107

---

Before submitting a PR mark the checkboxes for the items you've done (if you
think a checkbox does not apply, then leave it unchecked):

• The commits are consistent with our contribution guidelines.
• You've added tests (if possible) to cover your change(s). Bugfix, indentation, and font-lock tests are extremely important!
• You've run M-x checkdoc and fixed any warnings in the code you've written.
• You've updated the changelog (if adding/changing user-visible functionality).
• You've updated the readme (if adding/changing user-visible functionality).

Thanks!

[rrudakov]

Note: I didn't implement support of letfn and :let bindings for for and doseq. Probably will do it in a separate PR.

[bbatsov]

Fantastic work! 👏 👏 👏

[rrudakov]

That was fast :) Thanks!

[bbatsov]

And it will make the 0.5 release I was planning to cut tomorrow morning. :-)

[rrudakov]

And it will make the 0.5 release I was planning to cut tomorrow morning. :-)

Awesome!