[BUG] Make chroma match case-insenstive

- In the case that [go-enry](https://github.com/go-enry/go-enry/)
returned langauge doesn't match a lexer name (Either because its not
available or because it doesn't match Chroma's name), a last effort
attempt is made to use Chroma's matching.
- go-enry already applies `strings.ToLower` onto the filename to avoid
being case-sensitive, add the same code for Chroma's matching. The code
being used doesn't rely on the filename being case senstive for correct
matching.
- Adds unit test.
- Resolves #752

modules/highlight/highlight.go

@@ -96,7 +96,7 @@ func Code(fileName, language, code string) (output template.HTML, lexerName stri
 	}
 
 	if lexer == nil {
-		lexer = lexers.Match(fileName)
+		lexer = lexers.Match(strings.ToLower(fileName))
 		if lexer == nil {
 			lexer = lexers.Fallback
 		}
@@ -165,7 +165,7 @@ func File(fileName, language string, code []byte) ([]template.HTML, string, erro
 
 		lexer = lexers.Get(guessLanguage)
 		if lexer == nil {
-			lexer = lexers.Match(fileName)
+			lexer = lexers.Match(strings.ToLower(fileName))
 			if lexer == nil {
 				lexer = lexers.Fallback
 			}

modules/highlight/highlight_test.go

@@ -109,6 +109,12 @@ c=2
 			),
 			lexerName: "Python",
 		},
+		{
+			name:      "DOS.PAS",
+			code:      "",
+			want:      lines(""),
+			lexerName: "ObjectPascal",
+		},
 	}
 
 	for _, tt := range tests {