Add the fmt string formatting library

More info at http://fmtlib.net/

This commit was based on @jroweboy's work on his spdlog branch, but with
modifications.

.gitmodules

@@ -22,3 +22,6 @@
 [submodule "cryptopp"]
     path = externals/cryptopp/cryptopp
     url = https://github.com/weidai11/cryptopp.git
+[submodule "fmt"]
+    path = externals/fmt
+    url = https://github.com/fmtlib/fmt.git

CMakeLists.txt

@@ -258,6 +258,7 @@ add_subdirectory(${INI_PREFIX})
 add_subdirectory(externals)
 
 option(DYNARMIC_TESTS OFF)
+set(DYNARMIC_NO_BUNDLED_FMT ON)
 add_subdirectory(externals/dynarmic)
 
 add_subdirectory(externals/glad)

externals/CMakeLists.txt

@@ -9,3 +9,4 @@ endif()
 
 add_subdirectory(cryptopp)
 
+add_subdirectory(fmt)