Flymake: add new flymake-show-diagnostics-at-end-of-line option

Flymake: add new flymake-show-diagnostics-at-end-of-line option

Some editors have this.  Depending on your preference, this can either
be wildly distracting and easily confused with actual code, or a
significant early aid that relieves you from moving around or reaching
for the mouse to consult an error message.  To be safe, hide this
behind a customization variable and keep it disabled.

Personally, I find it less obstrusive and more helpful than expected.

* lisp/progmodes/flymake.el (flymake--delete-overlay): New helper.
(flymake--highlight-line): Handle flymake-show-diagnostics-at-end-of-line.
(flymake--clear-foreign-diags): Use flymake--delete-overlay.
(flymake--publish-diagnostics): Use flymake--delete-overlay.
(flymake-mode): Use flymake--delete-overlay.
(flymake-error-echo)
(flymake-warning-echo, flymake-note-echo): New faces.
(flymake-show-diagnostics-at-end-of-line): New option.
(Version): Bump to 1.3.4

* doc/misc/flymake.texi:
(Finding diagnostics): Mention flymake-show-diagnostics-at-end-of-line.
(Customizable variables): Mention
flymake-show-diagnostics-at-end-of-line and a few more relevant faces.

* etc/NEWS (Flymake): Mention flymake-show-diagnostics-at-end-of-line.