Update visible region with insert/erase delta

Adding a callback to the current views buffer allows the visible region
to update based on the delta of inserted/erased characters. This
prevents characters disappearing at the bottom right of the visible
region when typing.
Use https over git to clone submodules.
Merge pull request #238 from vivekgalatage/patch-1

Use https over git to clone submodules.
Merge pull request #235 from Dparker1990/update_visible_with_delta

Update visible region with insert/erase delta
Ignore dirty submodules

Ignore changes in submodules caused by the build process. This prevents
these submodules from showing up when checking git status.
Merge pull request #236 from Dparker1990/ignore_submodule_changes

Ignore dirty submodules