Code colors (backend support) (#590)

* testing code highlight with bfchroma render

* add bfchroma vendoring

* render code htm with classes

* adjust bfchroma test cases inside formatter

* go mod tidy

* fix markdown code render tests

* adjust comment text sanitizer to permit code HTML classes

* go mod tidy

* add chroma classes list regexp

* unify gitignore entries style

Co-authored-by: Umputun <umputun@gmail.com>

backend/_example/memory_store/go.sum

@@ -14,10 +14,20 @@ cloud.google.com/go/storage v1.0.0/go.mod h1:IhtSnM/ZTZV8YYJWCY8RULGVqBDmpoyjwiy
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@@ -30,6 +40,8 @@ github.com/chzyer/test v0.0.0-20180213035817-a1ea475d72b1/go.mod h1:Q3SI9o4m/ZMn
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@@ -39,6 +51,8 @@ github.com/didip/tollbooth v4.0.2+incompatible h1:fVSa33JzSz0hoh2NxpwZtksAzAgd7z
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@@ -121,6 +135,7 @@ github.com/mattn/go-colorable v0.0.9/go.mod h1:9vuHe8Xs5qXnSaW/c/ABM9alt+Vo+STaO
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+github.com/mattn/go-isatty v0.0.9 h1:d5US/mDsogSGW37IV293h//ZFaeajb69h+EHFsv2xGg=
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@@ -145,6 +160,7 @@ github.com/rogpeppe/go-internal v1.3.0/go.mod h1:M8bDsm7K2OlrFYOpmOWEs/qY81heoFR
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@@ -248,6 +264,7 @@ golang.org/x/sync v0.0.0-20190423024810-112230192c58/go.mod h1:RxMgew5VJxzue5/jJ
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