implement export bookmarks to json file from settings window

[artemgrygor]

Hey guys,
I've started with export/import functionality #75. I've added an export link to the settings window. @jamiewilson probably we should change UI here. exportBookmark.tmp.html contains all html for this link. Probably somewhere here should be an import link too.

@outcoldman for now I'm just saving bookmarksStorage.getAll function result as json file. What do you think about this? Chrome saves everything as html file. Would it be better for us to do the same with small modification?
P.S. it's just an init commit for discussion.

[outcoldman]

@artemgrygor nice, thank you for doing this.

My thoughts on this:

1. The most important: user should be able somehow import everything back.
2. It will be good to reuse some standard format, if there is some. Like https://msdn.microsoft.com/en-us/library/aa753582(VS.85).aspx Don't know if we can inject information about tags in it.

[outcoldman]

@artemgrygor it seems that it is possible to inject TAGS in Netscape format https://gist.github.com/jgarber623/cdc8e2fa1cbcb6889872

[artemgrygor]

@outcoldman just to clarify, am I right that for chrome bookmark folder tag.custom = false and for user created one `tag.custom = true'.
I have export bookmarks as Netscape format. Now I want to save exactly as chrome with folders.

[outcoldman]

@artemgrygor yep, you are right. As I remember custom = true are user created tags.