Add `default` argument to Map's value(_)

[emrcftci]

Description

• Add default value as an argument to Map's value(_)

We can assign default value effortlessly with this new feature.

Motivation and Context

• We will be use default argument for assign default value,

• We will avoid using parentheses for assign default values

  after this pull-request merge.

What kind of change does this PR introduce?

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] File changes for improvement
• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update
• [x] I have run tests

[emrcftci]

Could you please check? @tristanhimmelman

[tristanhimmelman]

I'm ok with this PR, but you will need to rebase master and get the tests passing again in Travis

[emrcftci]

I'm ok with this PR, but you will need to rebase master and get the tests passing again in Travis

I've rebased master into my branch but I accidentally take some commits from my other pull-request(#1081). So I've reverted these unwanted commits. Sorry for the mistake.

[tristanhimmelman]

No worries @emrcftci! Thanks for the contributions