Insert functions as snippets

[kattrali]

This changeset adds snippet support to function or method completion items so that an editor will insert tab stops for parameters. I need to go back through the spec to ensure I haven't missed anything, but I wanted to open it up so others could take a look and so far for my uses it works well.

Changes

• Added insertTextFormat to completion items, using Plain for all items except functions and methods, which use Snippet
• Modified the insertText for function/method items to insert parens and use tab stops for each parameter. For example, given the initial text math.fm, completing the item described as math.fmod(x, y) inserts math.fmod(${1:x}, ${2:y}) instead of math.fmod.

Screenshots

Before:

After:

[coveralls]

Coverage increased (+0.1%) to 80.164% when pulling 74d593dad3fae55b07bc1a8ae3a3034bcc422a87 on kattrali:kattrali/insert-fns-as-snippets into 905e71f9a97ea6366deb386503f9d976c87d9bab on Alloyed:master.