Extend snippet_from_file magic

In v0.4.0, this magics looks in the file for CELL_CONTENT_TO_SUBSTITUTE keyword and replace it by the code in the cell.

This requires the user to know the expected keyword - even if it can easily be found by trying and error message would tell it is missing.

However, we can't do two substitutions this way: for instance if we want to replace both a declaration and a definition of a method, we're screwed (or we must fuse them both in a same place).

Present issue aims to loosen the requirements and enable declaring several substitutions in the same cell:

    // cppmagics-replace GET_FOO_DECLARATION
    ... (code to substitute to GET_FOO_DECLARATION) in the file for which snippet is provided)...

    // cppmagics-replace GET_FOO_DEFINITION
    ... (code to substitute to GET_FOO_DEFINITION) in the file for which snippet is provided)...