Add props page-mode Mergeable

[Stormtv]

I took #80 and merged it with current version of vue-virtual-scroll-list

[codecov-io]

Codecov Report

Merging #103 into master will increase coverage by 3.49%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   93.47%   96.96%   +3.49%     
==========================================
  Files           1        1              
  Lines         184      198      +14     
==========================================
+ Hits          172      192      +20     
+ Misses         12        6       -6

Impacted Files	Coverage Δ
src/index.js	96.96% <100%> (+3.49%)	⬆️

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[Stormtv]

I tried to get the coverage higher but I'm not an expert with Jest unit testing. I couldn't figure out the proper way to emulate window events.

[tangbc]

I seem that demo of pagemode still has obvious bugs, such as:

This component calculates the rendering area by fixed container height, if using window, it also need
calculate with window height, it should be trouble.

[Stormtv]

Okay I must of missed that I'll take a look again and fix it.

[tangbc]

Run npm run demo:dev is the develop env of demos in your local.

[Stormtv]

I think I fixed the issue you were describing, if I missed something please let me know. I appreciate the feedback!

[tangbc]

Thanks, but busy these days, I will check when I'm free.