environment / variables for collections

[hrishix6]

• each collection should have ability to set some variables.
• user should be able to refer to these variables in request forms (url , headers, auth, query).
• highlighting should be there to indicate that variable is detected.

[hrishix6]

add support for saving, editing and inserting variables for collection.

[hrishix6]

substituting variables when making request is in progress.

[hrishix6]

this is done now , used mustache as substitution library.