Lowercase keywords fixer
feature #354 Lowercase keywords fixer (keradus)

This PR was squashed before being merged into the 0.4.x-dev branch (closes #354).

Discussion
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Lowercase keywords fixer

New Fixer for change keywords to lowercase version (PSR2 ¶2.5), e.g. AS => as

Commits
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02c44f5 Lowercase keywords fixer
fixed CS