Add JPG image support (#23)

## Summary

- Add basic JPG image support
- Map JPG back to 2-bit BMP output
- Can be used to later render the BMP file from disk or directly pass to
output if wanted
- Give the 3 passes over the data needed to render grayscale content,
putting it on disk is preferred to outputting it multiple times

## Additional Context

- WIP, looking forward to BMP support from
https://github.com/daveallie/crosspoint-reader/pull/16
- Addresses some of #11

lib/JpegToBmpConverter/JpegToBmpConverter.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <FS.h>
+
+class ZipFile;
+
+class JpegToBmpConverter {
+  static void writeBmpHeader(Print& bmpOut, int width, int height);
+  static uint8_t grayscaleTo2Bit(uint8_t grayscale);
+  static unsigned char jpegReadCallback(unsigned char* pBuf, unsigned char buf_size,
+                                        unsigned char* pBytes_actually_read, void* pCallback_data);
+
+ public:
+  static bool jpegFileToBmpStream(File& jpegFile, Print& bmpOut);
+};