Fix for imshow multiple definition error #48

When multiple files include matplotlibcpp.h
and another file load these files in the same time,
compile error of multiple definition in imshow occurs.
This fix add inline modifier to imshow().

matplotlibcpp.h

@@ -579,7 +579,7 @@ bool hist(const std::vector<Numeric>& y, long bins=10,std::string color="b",
 
 #ifndef WITHOUT_NUMPY
     namespace internal {
-        void imshow(void *ptr, const NPY_TYPES type, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords)
+        inline void imshow(void *ptr, const NPY_TYPES type, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords)
         {
             assert(type == NPY_UINT8 || type == NPY_FLOAT);
             assert(colors == 1 || colors == 3 || colors == 4);
@@ -607,12 +607,12 @@ bool hist(const std::vector<Numeric>& y, long bins=10,std::string color="b",
         }
     }
 
-    void imshow(const unsigned char *ptr, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords = {})
+    inline void imshow(const unsigned char *ptr, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords = {})
     {
         internal::imshow((void *) ptr, NPY_UINT8, rows, columns, colors, keywords);
     }
 
-    void imshow(const float *ptr, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords = {})
+    inline void imshow(const float *ptr, const int rows, const int columns, const int colors, const std::map<std::string, std::string> &keywords = {})
     {
         internal::imshow((void *) ptr, NPY_FLOAT, rows, columns, colors, keywords);
     }