Add character option to Indentation linter

Allow the `Indentation` linter's character to be specified as a space
(default) or tab.

Change-Id: I1e832735551182c3302f16d13229a375c2db3f9d
Reviewed-on: http://gerrit.causes.com/40779
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Add specs for tab indentation checking

Change-Id: I3862b950c3f26049ad18c343bbe8b4d4cb087057
Reviewed-on: http://gerrit.causes.com/40780
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Add detection of mixture of tabs/spaces

Previously, the `Indentation` linter would not report an error when tabs
were mixed with spaces, so if a line started with the right number of
appropriate characters (e.g. 1 tab), it could be followed by any number
of spaces and still pass the check.

Fix this problem by checking for a mixture of tabs and spaces and
reporting the problem.

Change-Id: I9503eafa40333887014f0649163f43a0f7bc6263
Reviewed-on: http://gerrit.causes.com/40781
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>