improve -help text

[VladimirAlexiev]

Plantuml -help returns a very useful description of command line options.

However @arnaudroques it can take some improvement to make it more readable.

• line-up all descriptions vertically. The longest option is -encodesprite 4|8|16[z] "file" and is not too long
• use just 2 spaces before options
• remove the parasitic word "To" from descriptions
• start descriptions with lowercase for consistency

Example showing all these points:

 -checkmetadata                  skip PNG files that don't need to be regenerated
 -p[ipe]                         use stdin for PlantUML source and stdout for PNG/SVG/EPS generation
 -encodesprite 4|8|16[z] "file"  encode a sprite at gray level (z for compression) from an image
 -computeurl|-encodeurl          compute the encoded URL of a PlantUML source file
 -decodeurl                      retrieve the PlantUML source from an encoded URL

• in this option, two \t are shown instead of lining-up the description:

    -theme xxx\t\tTo use a specific theme

• use a single tab to line-up descriptions (from the above item it appears you use 1 or 2 tabs)

[VladimirAlexiev]

Thanks @The-Lum ! I forget in 2 days, but github and the crowd remember forever :-)
Should we keep this open until your comment is fixed: #1753 (comment)