Move constructor support for Writer

[CreoValis]

As internal::Stack already has a move constructor, it's trivial to write one for Writer.
(Plus, i needed it for a project of mine 😄 .)

[coveralls]

Coverage decreased (-0.002%) to 99.935% when pulling 49540ac on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[coveralls]

Coverage remained the same at 99.937% when pulling 21a6676 on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[miloyip]

Please add unit tests for code coverage.
Please try to imitate the coding style.
And also, should PrettyWriter need the same thing?
Thank you.

[coveralls]

Coverage remained the same at 99.937% when pulling 2b73169 on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[CreoValis]

Thanks!
I've improved on the code style, and added move ctor support for PrettyWriter. Will add tests soon.

[coveralls]

Coverage decreased (-0.002%) to 99.935% when pulling 878eef8 on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[coveralls]

Coverage remained the same at 99.937% when pulling a5e6ba2 on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[coveralls]

Coverage remained the same at 99.937% when pulling a5e6ba2 on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[coveralls]

Coverage decreased (-0.002%) to 99.935% when pulling cf18d90 on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[coveralls]

Coverage increased (+8.0e-05%) to 99.937% when pulling 1a64cd0 on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[coveralls]

Coverage increased (+8.0e-05%) to 99.937% when pulling 1a64cd0 on CreoValis:writer-move-ctor into 5268211 on miloyip:master.

[CreoValis]

I've added unit tests for both classes' move constructors, and both CI tests pass now.
Is the code style acceptable in it's current form?

[miloyip]

Thank you