small refactor and correction

these are small change in preparation of my feature adding, all
explained here with there rational.

- FilterItems.ts

refactor of `getFiltered` and `getFilteredOut` so they share the same logic
code for filtering. this prevent error code duplication and error going
with it. also added some doc comment to better distinguish both
function.

I also maked these two function a bit purer by moving the searchTerm to
an optional parameter instead of passing by a shared object. this is a
better coding practice and permit to get filtered with a different
search term without changing the options

small refactor of `shuffle`. there was no reason to call `getFiltered`
twice, since `getFiltered` return a new array each time
same for the call of `slice()`, since a map alway return a new array.
the do-while was followed by a call to "shuffle" in an empty block, so you checked if the
new shuffle is different from the last, but shuffled one last time after
that. I removed it.

the `shouldBeFiltered` function have a misleading name, depending what a
"filtered" element mean for you (it is the element that are kept, or the
element that are removed?). I refactored it to use typescript "guard"
feature, removing the need of using explicit casting (that is error
prone). I also checked for "all" direcly in it, so we cannot forget to
check for it before calling the function.

- utils/shuffle.ts

I added typescript generic to keep the type information of the return
type the same as the one of the parameter.
I also corrected a small comment error. array.slice() do a shallow
clone, not a deep clone, and this is what we want.

- Filterizr.ts and Filterizr.test.ts

updated with the change of parameter of getFiltered

- tsconfig.json

added "lib" : ["es2016", "esnext", "dom"] to compiler option. this
removed some error found by my VisualStudio code intellisence of element
not found. by default, lib is only es2015+dom, and the library use some
2016 and next feature.

Author: Felix Brunet