Support inserting argument placeholders when completing function name

[matangover]

When completing a function, it would be nice to add argument placeholders. This is already supported by several language servers, like TypeScript, Dart, and Go, using a preference. See microsoft/vscode#34641.

[jakebailey]

I'm going to close this in favor of two issues targeting the specific parts of this, #37, which covers adding () to callables, and #221, which covers placeholder arguments specifically. The latter will take some specifying and consideration as to it being the default.