feat(lib): add getInitialValue() function

[manudss]

To be able to obtain the initial value of the form.

Closes #15

PR Checklist

Please check if your PR fulfills the following requirements:

• [x ] The commit message follows our guidelines: https://github.com/ngneat/forms-manager/blob/master/CONTRIBUTING.md#commit
• [x ] Tests for the changes have been added (for bug fixes / features)
• [x ] Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[x ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #15

What is the new behavior?

add a function to get the initial value of a form.

Does this PR introduce a breaking change?

[ ] Yes
[x ] No

[NetanelBasal]

I think we can make it short and clear. Just write the two functions without the whole explanation around it.

[manudss]

juste like this : ?

setInitialValue(name, value) - Set the Initial Value

formsManager.setInitialValue('form', initialValue);

getInitialValue(name) - Get the Initial Value or undefined if not exist.

formsManager.getInitialValue('form');

[NetanelBasal]

Yes