Make xolox#misc#os#exec() report exit codes (breaks backwards compatibility)

Working towards fixing this issue: xolox/vim-lua-ftplugin#8
Improve reporting of multi-line messages (now split over lines)
Bug fix for xolox#misc#option#eval_tags() option
Apply hard wrapping to README.md
Bug fix: Make xolox#misc#os#exec() compatible with (t)csh (reported by Armin Widegreen)

Original report:
  https://github.com/xolox/vim-easytags/pull/48
Improve error reporting in xolox#misc#os#exec()
Bug fix: Correct wrong expansion order in xolox#misc#path#which()
Added inline documentation everywhere, also included in README.md*

* Using a new tool I'm working on: A Python script that scans a Vim
  script project for global function definitions and their documentation
  comments (in Markdown format) and generates a single Markdown document
  that provides an overview of the functions defined in the scripts.
Improve the error message thrown by xolox#misc#compat#check()