Nit: add BarStartEmpty and BarEndFilled theme characters

[janpfeifer]

hi, and first many thanks for the wonderful package!

Recently I configured my progressbar as follows:

		progressbar.OptionSetTheme(progressbar.Theme{
			Saucer:        "[green]\uEE04[reset]", // 
			SaucerHead:    "[green]\uEE04[reset]", // 
			SaucerPadding: "[green]\uEE01[reset]", // 
			BarStart:      "[green]\uEE03[reset]", // 
			BarEnd:        "[green]\uEE02[reset]", // 
		}),

With the unicodes for progressbars (supported by all nerdfonts.com fonts), and I noticed that there is not way to make a distinction between the bar start/end filled or empty. The characters are as follows:

I was wondering if it would be possible to support those ?

cheers

[schollz]

@janpfeifer great idea, would welcome the pr!

[janpfeifer]

Done I think -- if u have a chance, pls check it out #204

I took the liberty of adding a couple of predefined Themes that the user can choose from if they want. It would be cool to have a more comprehensive selection at some point, I suppose ...

Here how it looks with the PR:

[schollz]

@janpfeifer this is so cool!!! what a huge improvement, I love how you incorporated them.

to get the nerdfonts working, which nerdfonts are needed? there seems to be a lot: https://www.nerdfonts.com/

[schollz]

Nevermind! I got it - it's here: https://github.com/ryanoasis/nerd-fonts/releases/download/v3.2.1/NerdFontsSymbolsOnly.zip

I'll make a little addition to the readme, this is so cool!

[janpfeifer]

Thanks @schollz ! Yes, any of the nerd fonts will do. And Fira Code seems to have incorporated the symbols as well.

[janpfeifer]

I just incorporated progressbar 3.16.1 into my project (still in a branch, but to be released soon), and it's working perfectly, thanks!

Closing the issue.