Fix issue #341 (github).
Ignore Errors E403 from included syntax files

This will silently discard Errors raised by syntax files that
are not designed to be included more than once.
Merge pull request #349 from m42e/master

Ignore Errors E403 from Included Syntax Files
Add test case for #341 (github).
Variable rename and illustration.
Update document for priority based clearsnippets.
Implement priority-based clearsnippets.
Code cleanup.

Full tested and does not break and old tests.
Refactor: snippet dictionary does not keep the extend info now.
Add/modify test cases for priority-based clearsnippets.
Some refinements.

1. Add docstrings.
2. Re-implement get_deep_extends().
3. Remove all inners.
Merge pull request #348 from cwahbong/select-cr-fix

Fix issue #341 (github).
Implements clearsnippets by priority.
Updates the doc for the new clearsnippets behavior. Slight rewording by me.