Generated Sass icon map can be corrupted

[sveetch]

Sometime a icon item from icomoon can have an invalid name like foo, bar because of its tags, and it should be something like foo, bar or foobar but never something we invalid characters for a Sass variables (no space, no ,, no $, etc..).

At least, the command line should validate each name and raise a warning or a blocking error when there is some invalid name.

This is to avoid unexpected issues with generated Sass map for users which forgot about the valid variable name requirement.

[sveetch]

Also it would be worth to mention about this requirement in documentation.

[sveetch]

Note: From Icomoon interface, we can see invalid name from a project in the tab "Generate SVG & more", the name is aside the displayed glyph for each icon.