Updating lastmod date string format to match W3C spec

[omardelarosa]

Issue

The lastUpdate value sets single digit day and month values. For example, today would be 2017-1-3 in the xml sitemap output. This causes sitemaps to be invalid on certain search engines, such as Google.

Solution

The sitemaps.org protocol mentions that the <lastmod> tag should work as such:

The date of last modification of the file. This date should be in W3C Datetime format. This format allows you to omit the time portion, if desired, and use YYYY-MM-DD.

Description

• Change just adds options to the GenerateDate function's call to toLocaleDateString that should output the correct, Goolge-friendly format now. 😄

[schneidmaster]

Hey @omardelarosa, good catch & thanks for the PR. Would you mind also bumping the package.json version to 0.1.2 & then I'll merge and release?