NEW Vim like keyshortcuts in HelpBrowser

[paum3]

Purpose and Motivation

Users of fantastic scnvim plugin in neovim can choose pandoc version of helpfiles, or classic style, which uses HelpBrowser class. For those, which are using a classic helpdoc this PR adds some VIM like keystrokes for comfortable browsing documentation. I've found it very useful and handy, code is tested & working on Linux.

the new keys are:

• h,j,k,l - back histo,down, up, forward histo
• ctrl + D, ctrl + U - bigger step scroll down, up
• g,G - go to top, to bottom of document
• J,K - zoom out, zoom in
• F3 - go to main Search page
• F5 - reload page
• / - search page
• alt + leftArrow, alt + rightArrow - back histo, forward histo

Types of changes

• New feature

To-do list

This needs to be checked on other than Linux platforms.

• Code is tested
• All tests are passing
• Updated documentation
• This PR is ready for review

[joshpar]

This looks good! A couple of suggestions that I'd like to see, but if someone else approves without the change I won't be heartbroken