Unit management

If no unit is set in config, nor in HA entity, do not print undefined and print nothing as unit

Improving

• Fix vertical alignment
• Updated readme
• Add decimal option

Thx @Luuk3333

Fix for gauge color

Merge pull request #10 from SNoof85/SNoof85-patch-5

Fix gauge-a background color

Fix for background

Merge pull request #9 from SNoof85/SNoof85-patch-4

Fix background color with default theme

Initial version

0.1

Make min_entity and max_entity optionnal