[Q&A] add tool to check for common spelling mistakes in source code
[Q&A] add tool to check for common spelling mistakes in source code

Add Makefile target `make codespell` to check for common spelling
mistakes in the source code, Markdown files, man pages, configure.ac
and Makefile.am. Makefile target `make qa` runs it too.

To make it work, please install codespell from:
https://github.com/lucasdemarchi/codespell

Preparation work for bug #1465012

didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc,didier-barvaux/rohc