Support custom encoding when writing BibTeX files

[amcar-dtu]

Add encoding parameter to file writing function.

Solves the UnicodeEncodeError at write time.

Error Stacktrace

Traceback (most recent call last):
  File "c:\Users\usr\documents\scripts\bib_parser.py", line 103, in <module>
    clean_bib()
    ~~~~~~~~~^^
  File "c:\Users\usr\documents\scripts\bib_parser.py", line 98, in clean_bib
    bibtexparser.write_file(bibtex_file, bibtex_database)
    ~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:\Users\usr\AppData\Local\miniconda3\Lib\site-packages\bibtexparser\entrypoint.py", line 160, in write_file
    f.write(bibtex_str)
    ~~~~~~~^^^^^^^^^^^^
  File "C:\Users\usr\AppData\Local\miniconda3\Lib\encodings\cp1252.py", line 19, in encode
    return codecs.charmap_encode(input,self.errors,encoding_table)[0]
           ~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
UnicodeEncodeError: 'charmap' codec can't encode character '\u2010' in position 2731: character maps to <undefined>