Extract the render-to-string portion of to_html() into its own public method

[Xymist]

Add a .to_inline_html() method to Plot which can then be sent elsewhere for rendering.

For example, when using in Evcxr this turns this construction:

let plotly_file = "/tmp/plot.html";
plot.to_html(plotly_file);
let plotly_contents = fs::read_to_string(plotly_file).unwrap();
println!("EVCXR_BEGIN_CONTENT text/html\n{}\nEVCXR_END_CONTENT", plotly_contents);
fs::remove_file(plotly_file)?;

into this one-liner:

println!("EVCXR_BEGIN_CONTENT text/html\n{}\nEVCXR_END_CONTENT", plot.to_inline_html());

I debated adding a helper method for that wrapping - I'm happy to do so but I feel it would be presumptive without an OK from @igiagkiozis since they might not wish to officially support such usage, and interested parties probably are using DARN anyway which is a better place for it.

Closes #12 as long as people are happy to do their own root div or Evcxr content wrapping.

[shahinrostami]

@Xymist I just found this whilst looking to introduce some improvements for DARN... I want to make some changes to reduce notebook sizes (sometimes 6-7MB) to 10-100KB. I'm torn between forking igiagkiozis/plotly and making these improvements directly or introducing them as a post-processing step in DARN.

[igiagkiozis]

Hi @Xymist, please re-direct the pull request to the dev branch.

[Xymist]

@igiagkiozis - Done, thank you.

[igiagkiozis]

@Xymist I just found this whilst looking to introduce some improvements for DARN... I want to make some changes to reduce notebook sizes (sometimes 6-7MB) to 10-100KB. I'm torn between forking igiagkiozis/plotly and making these improvements directly or introducing them as a post-processing step in DARN.

@shahinrostami the size is due to every plot being self contained, which means that plotly.js is copied in every single plot. This is quite inefficient for this use case. I'm currently looking into alternative ways to better support this use case.

[igiagkiozis]

@Xymist, @shahinrostami I've pushed an implementation that works with inline html nicely without the extra js dependencies. You can try it out in the latest dev branch; this feature will be released in version 0.5.xx