Issue title: Get execution time
 Issue body: Hi,

How to get the execution time of each query in browser?

 Comments: 
 Comment 0: Are you running sync or async?

 Comment 1: async
 Comment 2: ```
var start = new Date();

alasql('SELECT 1+2 AS a', [], function(){

    console.log("Script took", new Date() - start)
})

```
{'source': 'GithubIssues', 'repo_name_with_owner': 'AlaSQL/alasql', 'issue_id': 111355901, 'repo_stars': 6004.0}