Get build and restore options from environment; provide set-quiet target
- see #123
- leaves default build unchanged
- run `build set-quiet default` for a much-quieter build
Address PR comments
- "K_build_options" -> "KPM_build_options" environment variable
- "K_build_options" -> "KPM_restore_options" (oops)
- "set-quiet" -> "--quiet" target

nit: remove trailing whitespace
Updating commands for non windows machines
Add IsTeamCity flag
Remove some repos
Fix #118, Only check K projects for XML documentation errors
- ignore leftover XML files e.g. from other branches
 - tested manually in MVC repo with a bad doc file outside a K project
- limit checked XML files to those in `bin` directory under K projects
- align with clean target: that only removes bin directories under K projects
- approach ignores XML files in .csproj projects but no such projects in
  repos where _k-xml-docs-test.shade file is used

Address old [comment](https://github.com/aspnet/Universe/pull/101/files#r16633938) from @Eilon on PR#101
> Traditionally we use i++ and not ++i.
Add the complete build option to the build script (build + test)