feature: Added '--specignore' option to pass file containing specs to be ignored for fission spec commands (#2239)

- New flag `--specignore` has been added for command fission spec [list | validate | apply | destroy]
- This flag is optional.
- The default spec ignore file name is `.specignore`, if the flag is not used
- The spec ignore file existence will be checked in the `--specdir` path
- If spec ignore file is passed to the flag, with a name other than .specignore and if it does not exist in the `--specdir` path then an error will be returned.
- `--specdir` will be the root path against which the patterns will be evaluated
- The behaviour of the spec ignore file will be similar to .gitignore (only for yml and yaml files). For reference: http://git-scm.com/docs/gitignore

go.sum

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