Add option to export JSON, change file handling

No longer serialize into strings, instead use Vec<u8> as it is
what we're writing to the files.

Fixup comments

Cargo.toml

@@ -18,6 +18,7 @@ cfg-if = "0.1.2"
 csv = "1.0.0-beta.5"
 serde = "1.0.33"
 serde_derive = "1.0.33"
+serde_json = "1.0.11"
 
 [target.'cfg(not(windows))'.dependencies]
 libc = "0.2"