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Add support for .env file. Close Issue zerasul#32.

This Pull Request adds support to define blask settings using a .env
file.

**Usage**

Add an .env file in the root folder defining the variables FLASK_APP,
templateDir, postDir, defaultLayout, staticDir, title.
(Use .env.example as template)

`$ pipenv shell` will launch the virtualenv and automatically create
environment variables from .env

**Behaviour**

In order to ensure backwards compatibility, the following precedence is
considered to define blask settings:

1. if the environment variable BLASK_SETTINGS exists, the settings are
   loaded from this module (e.g. settings.py)
2. otherwise, environment variables, and finally default values are used
3. in any case, whenever Blask() is called with arguments, they always
   take precedence

**Description of changes**

* blask/blasksettings.py - in `BlaskSettings.__init__()``, replace
  default settings with environment variables. Add clarifications to
  docstring and comments.

* blask/blaskcli.py - fix typos in comments and docstrings, minor code
  formatting

* tests/settings_test.py - temporarily delete environment variables
  before the tests and restore them afterwards.
  Add test_from_dotenv() to test with environment variables

All tests passed.

Close zerasul#32.