Feature: follow up of adding legend

[xhdix]

follow-up of #62

maybe we need:

• add edges details
• be able to turn it off
• be able to use pyvis' template
• ?

[RYNEQ]

About the changes in #62 now we have two identical templates which one of them is always used, the templates difference is some JS and CSS codes,

• For consistency reasons it will be better to merge these two into one template and use a template variable to add additional codes
• It also can be a good idea to have some other output types (multi-selectable by user), e.g. dotfile, SVG, png, pdf, ps, ...
• as a dotfile is a good common file format between graph analysis softwares it is better to make this type the bridge between other types (so we can have more options for generation, analysis and conversion of output types - e.g. we can use Graphvis or even Latex)