Single quotation mark in annotation causes loss of saved bookmarks

In `bm#serialize()`, the `content` field is escaped, but the`annotation` field is not. If the annotations contain single quotation marks (`'`), `exec join(a:data, " | ")` in  `bm#deserialize()` will fail silently,  and `l:bm_sessions` will be overwrite upon `VimLeave`, causing loss of saved bookmarks. Please fix this.
