text: Title: The 'silent' keyword argument does not behave pythonically in version 2.1.0
  Body: # Summary of your issue
  
  When I give `silent` as the last argument to `tabula.read_pdf()`, the mere existence of `silent` keyword causes it to be considered as `True`. To unsilence Tabula's `sys.stderr` output, I have to omit it. Setting `silent=False` or `silent=None` did not turn on Tabula's `sys.stderr` output.
  I did not test other Tabula functions or other `tabula.io.build_options()` options, but I suspect that the bug exists also for them.
  
  Workaround which worked for me:
  ```
  tabula.read_pdf(...., **({ "silent": True } if silent else {})
  ```
  which is not Pythonic.
  
  # Check list before submit
  
  <!--- Write and check the following questionaries. -->
  
  - [x] Did you read [FAQ](https://tabula-py.readthedocs.io/en/latest/faq.html)?
  
  - [ ] (Optional, but really helpful) Your PDF URL: ?
  Not PDF-specific.
  
  - [X] Paste the output of `import tabula; tabula.environment_info()` on Python REPL: ?
  >>> import tabula
  >>> tabula.environment.info()
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
  AttributeError: module 'tabula' has no attribute 'environment'
  >>> tabula.environment_info()
  Python version:
      3.8.2 (default, Apr  2 2020, 08:26:09) 
  [GCC 8.3.0]
  Java version:
      openjdk version "11.0.7" 2020-04-14
  OpenJDK Runtime Environment (build 11.0.7+10-post-Debian-3deb10u1)
  OpenJDK 64-Bit Server VM (build 11.0.7+10-post-Debian-3deb10u1, mixed mode, sharing)
  tabula-py version: 2.1.0
  platform: Linux-4.19.0-9-amd64-x86_64-with-glibc2.28
  uname:
      uname_result(system='Linux', node='c4', release='4.19.0-9-amd64', version='#1 SMP Debian 4.19.118-2+deb10u1 (2020-06-07)', machine='x86_64', processor='')
  linux_distribution: ('Debian GNU/Linux', '10', 'buster')
  mac_ver: ('', ('', '', ''), '')
  
  # What did you do when you faced the problem?
  I used the workaround:
  ```
  tabula.read_pdf(...., **({ "silent": True } if silent else {})
  ```
  
  ## Code:
  
  ```
  tabula.read_pdf(...)
  ```
  with the following values of `silent` option: 
  - option does not exist
  - `silent=True`
  - `silent=False`
  - `silent=None`
  
  ## Expected behavior:
  - missing option: messages about font errors are outputted.
  - `silent=True`: messages about font errors are NOT outputted.
  - `silent=False`: messages about font errors are outputted.
  - `silent=None`: messages about font errors are outputted.
  
  ## Actual behavior:
  - missing option: messages about font errors are outputted - OK
  - `silent=True`: messages about font errors are NOT outputted - OK
  - `silent=False`: messages about font errors are NOT outputted - NOT OK
  - `silent=None`: messages about font errors are NOT outputted - NOT OK
  
  ## Related Issues:
  The bug is cosmetic by its nature - does not affect real functionality and has an easy workaround.