text: Title: skipif mark can't utilize global python variables or vars returned by fixtures
  Body: Hi, I believe this is a feature request for the `skipif` mark. 
  
  ### **Issue**
  I'm trying out the `skipif` mark (see three code examples below), but the `eval()` function is only able to access vars stored in tavern.util.dict_util (ie. system environment variables and perhaps variables included in `!include` .yaml files). I tried `skipif: "global_python_var is True"` which uses a global var created in the conftest.py (also tried `skipif: "global_python_var in globals()"`). Additionally, I tried accessing variables returned from fixtures (also defined in the conftest.py) using the `skipif: '{var_name}'` format, but get the following error: 
  
  **ERROR:tavern.util.dict_util:Key(s) not found in format: url**, with this output (I set all env_values to None):
  ```
  {'tavern': 
     {'env_vars': 
        {'NVM_INC': None, 'LDFLAGS': None, 'TERM_PROGRAM': None, 'PYENV_ROOT': None, 'NVM_CD_FLAGS': None, 'TERM': None, 'SHELL': None, 'CPPFLAGS': None, 'TMPDIR': None, 'GOOGLE_APPLICATION_CREDENTIALS': None, 'VAULT_ADDR': None, 'TERM_PROGRAM_VERSION': None, 'TERM_SESSION_ID': None, 'PYENV_VERSION': None, 'NVM_DIR': None, 'USER': None, 'SSH_AUTH_SOCK': None, 'PYENV_DIR': None, 'VIRTUAL_ENV': None, 'PATH': None, 'LaunchInstanceID': None, 'PWD': None, 'LANG': None, 'PYENV_HOOK_PATH': None, 'XPC_FLAGS': None, 'XPC_SERVICE_NAME': None, 'HOME': None, 'SHLVL': None, 'PYTHONPATH': None, 'LOGNAME': None, 'NVM_BIN': None, 'SECURITYSESSIONID': None, '__CF_USER_TEXT_ENCODING': None }
     }
  }
  ```
  
  ### **Request**
  Could the eval function called by `skipif` access information just like the test that it is marking **and** the python global namespace? Additionally, utilizing skipif with external functions (ie. getting a function to return either "True" or "False", would also be a good alternative). My overall goal is to skip all tests if my basic health-check test failed following these steps:
  
  
  ### _My intended usage and test examples_
  1. run healthcheck/base test
  2. verify response with external function which will either create a global, or change an existing global created in conftest.py (returns True or False based on response)
  3. Other tests are skipped if the `skipif eval()` finds that the global var == False. (Alternatively, skips if external function called in eval() returns `"False"`)
  
  
  Here are the example code snippets I tried (located in test_name.tavern.yaml file):
  
  ```
  marks:
    - skipif: "'healthcheck_failed' in globals()"
  ```
  
  ```
  marks:
   - skipif: "'{autouse_session_fixture_returned_from_conftest}' is True"
  ```
  
  ```
  marks:
   - skipif:
       - $ext:
         - function: "utils:return_true" 
  ```