add license notice

• adds license notice for the data from an external project

icon font support

• supports icon font specification e.g.) :fa5_cat: hello
• fixes compile errors in MacOS about constexpr because of strlen use
• makes an item which has an empty msgstr ignored
• transpiled into C++ from C
• updates CLI interface

bugfix

• fixes compiler warnings about integer type conversion
• now buffer type taken by pack_header and unpack_header is changed to uint8_t* from char*
• makes custom command added by add_mulan depend to mulan executable

first release

v1.0.0

add LICENSE