Add basic comments support to the AST

[xmo-odoo]

Cf #14

Follows the Recast comment-attachment structure[0], though only implements a small subset in a straightforwardly hacky manner, and does not come close to implementing the entire comment-attachment structure or algorithm:

• comments preceding (some) nodes are extracted and set on the comments key of the host node
• does not extract the location
• only does leading comments (I don't think it'll mis-attach trailing comments but that's not impossible)
• may not work for all node types
• the comments key may be missing entirely
• uses esprima's type (Block/Line) rather than Babel's

[0] dev's reasoning about some comments being ambiguous in Esprima's structure/extension makes sense to me: https://github.com/jquery/esprima/issues/1024#issuecomment-74179681 https://github.com/benjamn/recast/issues/159#issuecomment-76054246

[PiotrDabkowski]

Thank you very much! Seems to work for now. Maybe I will update the pyesprima according to the newest esprima.js version if I find some time.