Fix some spelling errors - mostly in comments & documentation.

I did not touch the Changelog file and any third party stuff.

additonal -> additional (1)
apppend -> append (2)
constuctor -> constructor (2)
contect -> context (2)
dobulequote -> doublequote (1)
elemnts -> elements (1)
exakt -> exact (2)
exluded -> exclude (1)
explicitely -> explicitly (2)
narcissic -> narcissistic (1)
ocurred -> occurred (1)
occuring -> occurring (2)
preceeds -> preceded (1)
ot -> to (2)
wehther -> whether (2)

test/src/unit-regression.cpp

@@ -164,7 +164,7 @@ TEST_CASE("regression tests")
 {
     SECTION("issue #60 - Double quotation mark is not parsed correctly")
     {
-        SECTION("escape_dobulequote")
+        SECTION("escape_doublequote")
         {
             auto s = "[\"\\\"foo\\\"\"]";
             json j = json::parse(s);