Add nbFile overload for file embedding (#219)

* Add nbFile overload for file embedding

This also adds an example

* Update src/nimib.nim

Co-authored-by: Pietro Peterlongo <pietro.peterlongo@gmail.com>

---------

Co-authored-by: Pietro Peterlongo <pietro.peterlongo@gmail.com>

docsrc/allblocks.nim

@@ -110,13 +110,22 @@ nimibCode:
 
 nbCodeBlock: "nbFile"
 nbText: """
-`nbFile` saves the content of the block into a file. It takes two arguments: the name of the file and the content of the file.
+`nbFile` can save the contents of block into a file or display the contents of a file. 
+
+To save to a file it takes two arguments: the name of the file and the content of the file.
 The content can be a string or a code block.
 """
 nimibCode:
   nbFile("exampleCode.nim"):
     echo "This code will be saved in the exampleCode.nim file."
 
+nbText: """
+
+To display a file, it takes one argument: the file's path.
+"""
+nimibCode:
+  nbFile("../LICENSE")
+
 nbCodeBlock: "nbRawHtml"
 nbText: """
 Certain things are not doable with pure Markdown. You can use raw HTML directly with the `nbRawHtml` block.

src/nimib.nim

@@ -141,6 +141,13 @@ template nbFile*(name: string, body: untyped) =
     nb.blk.context["ext"] = name.getExt
     nb.blk.context["content"] = nb.blk.code
 
+template nbFile*(name: string) =
+  ## Read content from a file instead of writing to it
+  newNbSlimBlock("nbFile"):
+    nb.blk.context["filename"] = name
+    nb.blk.context["ext"] = name.getExt
+    nb.blk.context["content"] = readFile(name)
+
 when moduleAvailable(nimpy):
   template nbInitPython*() =
     import nimpy