Add tag & stats badge to the README

[wilzbach]

• The tag badge will automatically show the latest git tag
• The issue stats badge shows the time to close a PR and is intended to act as a positive motivation for contributors

Preview:

I might also make sense to put them in their own line now:

[coveralls]

Coverage increased (+0.01%) to 61.286% when pulling 930c0b5 on wilzbach:patch-1 into 0595542 on dlang:master.

[s-ludwig]

How about putting them just below the introductory paragraph (e.g. https://github.com/rejectedsoftware/vibe.d/)? That means it's still immediately visible, but the most important information stays together at the top.

[andralex]

@wilzbach ?

[s-ludwig]

@wilzbach: I've committed a change that adds the badges below the introductory text now. However, the issuestats badge shows "invalid" for me. Is there anything that needs to be done/broke since you opened the PR?