Implemented string interpolation for sysmsg and overhead

[Tranq1]

This PR enables string interpolation for variables in sysmessage and overhead outputs. To achieve this, put your variable in between double curly braces like this: {{myvar}}.

Example script:

@setvar myvar backpack
getlabel backpack mylabel

removelist mylist
createlist mylist
pushlist mylist "hello"

sysmsg "myvar: {{myvar}}" 40
sysmsg "mylabel: {{mylabel}}" 30
foreach listvar in mylist
    sysmsg "list var: {{listvar}}" 20
endfor
sysmsg "not found: {{doesntexist}}" 10

Result:

[anethus]

Beta Client Build and Release #55