Fix inconsistencies in documentation

[abravalheri]

As I am a lazy person, I bundled togheter 2 changes in only one PR. Please let me know if you want me to split it so the first one can be accepted without the second.

1. Fix extension flag on README (package instead of project)
2. In the README description, the following example is given:
   putup --custom-extension notebooks
   However this throws an error in the current implementation.
   The changes in this commit address that by adding the prefix by default and warning the user instead of raising an exception.

[coveralls]

Pull Request Test Coverage Report for Build 132

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+2.1%) to 96.129%

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Files with Coverage Reduction	New Missed Lines	%
.tox/default/lib/python3.6/site-packages/pyscaffoldext/custom_extension/extension.py	2	96.83%

Totals
Change from base Build 130:	2.1%
Covered Lines:	138
Relevant Lines:	142

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💛 - Coveralls

[FlorianWilhelm]

@abravalheri Thanks for your improvements!