clang-format-vsix: Add "Format on Save" feature

This change adds a feature to the clang-format VS extension that optionally
enables the automatic formatting of documents when saving. Since developers
always need to save their files, this eases the workflow of making sure source
files are properly formatted.

Differential Revision: https://reviews.llvm.org/D29221

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@299543 91177308-0d34-0410-b5e6-96231b3b80d8