Fix long names thumbnail

[LePetitTim]

• Do not use hash for SEO
• Truncate end of the file after generate thumbnail
• Add tests with long names

#388

[jrief]

Thanks for your pull request. After looking at your code, I would like to add my annotations:

• High resolution will go away, so there is no need to add tests for it explicitly.
• Why didn't you create an alternative namer function, say truncator and add it to easythubnails.namers. Then one can use either the new namer to create truncated names with a hash, if the source name is too long, or the existing default namer. This could help to prevent compatibility issues. What's your opinion?

[jrief]

Closing for inactivity.