Add timestamp token

[JonasBang]

Description

To cater for the following use cases, proposal is to add a timestamp option.

Use cases:

• Need to know exact build time of a build (e.g. to identify exact dependencies used at build time).
• Need to rebuild a commit (e.g. with newer resolved dependencies), and need to stamp the new build differently from the previous.

Proposed Solution

Add a timestamp token.

It should insert a 10 digit timestamp calculated in seconds since Unix Epoch, and prefix it with 't' (for timestamp).

Name: Timestamp
Token: {timestamp}
Where: label, metadata
Description: Inserts a ten digit timestamp calculated in seconds since Unix Epoch, prefixed with t (for timestamp)

Similar to output from PHP time() function.

Example

Version string with timestamp as metadata: 0.1.0-alpha2.5.c903782+t1525376494
Version string with timestamp as label: 0.1.0-alpha2.5.c903782.t1525376494