Support prefix

[chrisgoffinet]

Just like when defining User Snippets in VSCode, it would be helpful if this extension supported prefix.

{
    "Insert heading level 1": {
       "prefix": "heading1",
       "body": ["# ${1:${TM_SELECTED_TEXT}}$0"],
       "description" : "Insert heading level 1"
    }
}

The prefix property defines the trigger phrase for the snippet. It can be a string or an array of strings (if you want multiple trigger phrases). Substring matching is performed on prefixes, so in this case, typing "h1" would match our example snippet.