Make number of subjects variable

[shruti170901]

Ask the number of subjects and change the files accordingly

[SihotiyaKartik]

I want to work on this issue

[sujal111]

Please allow me to work on this issue

[shruti170901]

Great! I am assigning you both

[SihotiyaKartik]

I have updated my code. Please check Pull request #15
FEATURES ADDED

1. Number of subjects is variable
2. changed the css stylesheet
3. now the minimum value of cgpa to be entered is 0, previously user can enter a negative value also
4. added a navbar with title 'Cgpa Calculator'

LOOK BEFORE ADDING NUMBER OF SUBJECTS

LOOK AFTER ADDING NUMBER OF SUBJECTS