Feat/improve css compatibility

[matcornic]

Great news, a friend of mine has lent me a Windows with a licensed Outlook. So I can fix the Outlook compatibility issue.

So this PR contains code for:

• Fix outlook missing button Buttons not shown in Outlook Client #59
• Fix outlook quotes display
• Add CSS inlining by default with https://github.com/vanng822/go-premailer/premailer (now Outlook displays the text as centered)

=> Replaces PR #48

Hey @kauffj @loeffel-io @vjeantet @dmuriel @elliots 🙌
Can you test this PR on your side ?

It worked for me on a Windows 10 with latest version of Outlook. I think it's still perfectible, but it does the work.

Edit: the size of the button now adapts to content in default theme.

[codecov-io]

Codecov Report

Merging #61 into master will decrease coverage by 0.85%.
The diff coverage is 85.24%.

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
- Coverage   98.45%   97.59%   -0.86%     
==========================================
  Files           3        3              
  Lines         907      957      +50     
==========================================
+ Hits          893      934      +41     
- Misses          8       16       +8     
- Partials        6        7       +1

Impacted Files	Coverage Δ
default.go	100% <100%> (ø)	⬆️
flat.go	99.55% <100%> (+0.01%)	⬆️
hermes.go	68.18% <43.75%> (-9.6%)	⬇️

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[loeffel-io]

Great news! Thank you very much 🚀

[dmuriel]

Thank you very much. I just tested on Windows Server 2016 with Outlook 2016, and email is displaying correctly, except the button, as you said, "does not adapt to content, but it does the work."

[matcornic]

@dmuriel I will try this evening to adapt the button width from the length of the text (unless it reaches a maximum width). It should be technically doable, but we will see if it's not such a good idea.

[dmuriel]

@matcornic you can also check this workaround I made for Outlook only, maybe you can use something for the button. It may not be pretty but it worked for me while waiting for a solution.

[kauffj]

Asking someone else at @lbryio to look at this. Thank you!

[tzarebczan]

LGTM, thank you again for looking into and fixing this!

[matcornic]

@dmuriel The button width in default theme is now adapting to text content since 027481f

I checked with multiple text sizes and it seems to react nicely. I implemented the same behaviour with both classic/Outlook sections for compatibility purpose. max-width and min-width could have been used, but using only width keeps compatibility requirements quite low.

Let me know what you think about it :)

(last commit before merge 🚀)

[dmuriel]

@matcornic I agree with leaving only with the width attribute. Regarding the button width it seems that now is cropping the text.
"Outlook Web"
"Outlook 2016"

[matcornic]

@dmuriel weird, I used the trial of Litmus, and the button looks good on Outlook 2016.

I will merge as is.

[lingr7]

outlook 2016 windows10 has problem

[lingr7]

theme flat and default has the same problem ,can not show button

[lingr7]

I find a workaround. success show button

DisableCSSInlining: true,

	h := hermes.Hermes{
		// Optional Theme
		// Theme: new(Default)
		Theme:              new(hermes.Flat),
		DisableCSSInlining: true,