Fix typo in specifications
Mark up 'ENV' consistently in documentation
Use code blocks consistently in documentation

The documentation mixes both traditional code block syntax and fenced
code block syntax. Stick to the latter for consistency.
Mention 'dotenv' executable in documentation

The 'dotenv' executable is easy to miss yet useful in many situations.
Mention it in the documentation.

Following the example set by the changelog, refer to a Python script in
the documentation to underline the point that using Dotenv this way is
not specific to Ruby.
Merge pull request #92 from jvirtanen/features/dotenv-executable-documentation

Mention 'dotenv' executable in documentation
Merge pull request #91 from jvirtanen/fixes/typo

Fix typo in specifications