Add assertions completions

[kipit]

This PR provides completions for AVA assertions.

[sindresorhus]

Shouldn't it be in the same format as the other snippets? Meaning one file per snippet. I don't get the difference between snippets and completions. Would you mind elaborating? :)

[kipit]

Hum… That one feels more like an completion than a snippet. The main difference from a user perspective, is the completion choice popping up while user begin to type t.ok. Completion seems a good fit when the trigger belongs to the domain language.

But to be honest, you made me realize that they can be used interchangeably: the result can be the same.

So now, you make me realize that the other ava snippet could be best fit for completion, except for the ava snippet.

[sindresorhus]

One potential issue with using completion for this is that it won't trigger if the user has turned tab completion off. Snippets will work regardless.

From the default preferences:

    // When enabled, pressing tab will insert the best matching completion.
    // When disabled, tab will only trigger snippets or insert a tab.
    // Shift+tab can be used to insert an explicit tab when tab_completion is
    // enabled.
    "tab_completion": true,

[sindresorhus]

No reply for a while, so merging for now, but I'd still like to get to the bottom of this. #8

Thanks @kipit :)

[kipit]

@sindresorhus my bad :( I've perfectly heard your comment, but I am a bit busy ATM