Support leading comments for sequenceDiagrams

[ashsearle]

Fixes #141

[knsv]

Thanks! This one has been around for a while. Good to have it fixed!
Merging.

[MikeB2012]

Leading %% bug still seems to be an issue for gantt (in v6.0.0). Using the simple example at: http://knsv.github.io/mermaid/#styling40, symptoms and workaround are identical to issue #141 ie. remove the comment and everything works.

Error:

15:02:17 (847) :DEBUG: Parse error on line 1:
... Gannt Chart example gantt title A G
-----------------------^
Expecting 'gantt', got 'NL'
mermaid.min.js:24 15:02:17 (847) :WARN: TypeError: Cannot read property '0' of undefined
at t.parse (mermaid.min.js:22)
at Object.e.exports.draw (mermaid.min.js:22)
at C (mermaid.min.js:24)
at Object.n.render (mermaid.min.js:24)
at Object.c [as init] (mermaid.min.js:24)
at Object.n.contentLoaded (mermaid.min.js:24)
at mermaid.min.js:24