Release v0.4.0

• supported following box types.
  • btrt
  • cslg
  • frma
  • saio
  • saiz
  • schm
  • tenc
  • trep
  • 3GPP metadatas
  • metadatas under ilst
  • wave
  • mp4a (QuickTime)
• created Context struct.
• escape unprintable runes for string fields.
• fixed the issue where marshaller writes no bytes for 0 value of varint.
• fixed the issue with the rounding error of stringify for fixed point number.
• fixed the issue where colr box can not accept unknown colour type.
• any other tiny enhancements and fixes.