Added the option removecr to inputs.exec to remove all carraige returns
(CR, ASCII 0x0D, Unicode codepoint \u0D, ^M). The option is boolean and
not enabled if not present in the config file.

closes #1606

Updated CHANGELOG.md with information about removecr

Ran go fmt ./...

Moved removal of CRs to internal/internal.go

Moved the code to remove carriage returns from
plugins/inputs/exec/exec.go to internal/internal.go. Additionally
changed the conditional on which it gets applied from using a
configuration file option to checking if it is running on Windows.

Moved Carriage Return check to correct place

Moved the carriage return removal back to the exec plugin. Added unit
testing for it. Fixed a bug (removing too many characters).

Ran go fmt ./...

Reverted CHANGELOG to master

Updated Changelog