Change how word splitting works when completing

Previously, I relied on replxx to correctly split words based on
word-splitting characters. However, as completion gets more complex and
completions possibly insert word-splitting characters, it starts to do
weird stuff like deleting some of your input. Fortunately, replxx allows
you to set the context length for completion - that is, how many
character it should consider as part of the word you're completing.

Change-Id: I035ac5059c8ab125efedb90cbeb2910f20da04a7
diff --git a/src/ast_handlers.hpp b/src/ast_handlers.hpp
index 0ce611d..d99e020 100644
--- a/src/ast_handlers.hpp
+++ b/src/ast_handlers.hpp
@@ -337,6 +337,7 @@
     {
         auto& parserContext = x3::get<parser_context_tag>(context);
         parserContext.m_suggestions.clear();
+        parserContext.m_completionIterator = boost::none;
         parserContext.m_completionSuffix.clear();
     }
 };